Fix issues reported by coverity

*** CID 149289:  Null pointer dereferences  (REVERSE_INULL)
/slurm/src/slurmctld/step_mgr.c: 1993 in _step_dealloc_lps()
1987     	xassert(job_resrcs_ptr->cpus);
1988     	xassert(job_resrcs_ptr->cpus_used);
1989
1990     	if (step_ptr->step_layout == NULL)	/* batch step */
1991     		return;
1992
>>>     CID 149289:  Null pointer dereferences  (REVERSE_INULL)
>>>     Null-checking "job_resrcs_ptr" suggests that it may be null, but it has already been dereferenced on all paths leading to the check.
1993     	if (job_resrcs_ptr == NULL)
1994     		return;
1995     	i_first = bit_ffs(job_resrcs_ptr->node_bitmap);
1996     	i_last  = bit_fls(job_resrcs_ptr->node_bitmap);
1997     	if (i_first == -1)	/* empty bitmap */
1998     		return;

** CID 149288:  Resource leaks  (RESOURCE_LEAK)
/slurm/src/api/reconfigure.c: 171 in _send_message_controller()

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*** CID 149288:  Resource leaks  (RESOURCE_LEAK)
/slurm/src/api/reconfigure.c: 171 in _send_message_controller()
165     	}
166     	resp_msg = xmalloc(sizeof(slurm_msg_t));
167     	slurm_msg_t_init(resp_msg);
168
169     	if ((rc = slurm_receive_msg(fd, resp_msg, 0)) != 0) {
170     		slurm_shutdown_msg_conn(fd);
>>>     CID 149288:  Resource leaks  (RESOURCE_LEAK)
>>>     Variable "resp_msg" going out of scope leaks the storage it points to.
171     		return SLURMCTLD_COMMUNICATIONS_RECEIVE_ERROR;
172     	}
173
174     	if (slurm_shutdown_msg_conn(fd) != SLURM_SUCCESS)
175     		rc = SLURMCTLD_COMMUNICATIONS_SHUTDOWN_ERROR;
176     	else if (resp_msg->msg_type != RESPONSE_SLURM_RC)

** CID 149287:  Integer handling issues  (NEGATIVE_RETURNS)
/slurm/src/slurmdbd/backup.c: 65 in run_dbd_backup()

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*** CID 149287:  Integer handling issues  (NEGATIVE_RETURNS)
/slurm/src/slurmdbd/backup.c: 65 in run_dbd_backup()
59     	primary_resumed = false;
60
61     	memset(&slurmdbd_conn, 0, sizeof(slurm_persist_conn_t));
62     	slurmdbd_conn.rem_host = slurmdbd_conf->dbd_addr;
63     	slurmdbd_conn.rem_port = slurmdbd_conf->dbd_port;
64     	slurmdbd_conn.cluster_name = "backup_slurmdbd";
>>>     CID 149287:  Integer handling issues  (NEGATIVE_RETURNS)
>>>     Assigning: "slurmdbd_conn.fd" = a negative value.
65     	slurmdbd_conn.fd = -1;
66     	slurmdbd_conn.shutdown = &shutdown_time;
67
68     	slurm_persist_conn_open_without_init(&slurmdbd_conn);
69
70     	/* repeatedly ping Primary */

** CID 149286:  Integer handling issues  (INCOMPATIBLE_CAST)

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*** CID 149286:  Integer handling issues  (INCOMPATIBLE_CAST)
/slurm/src/common/slurmdbd_defs.c: 3096 in slurmdbd_unpack_init_msg()
3090
3091     	*msg = msg_ptr;
3092
3093     	/* We don't use rollback going forward and version was packed after it
3094     	 * unfortunately */
3095     	if (rpc_version < SLURM_17_02_PROTOCOL_VERSION)
>>>     CID 149286:  Integer handling issues  (INCOMPATIBLE_CAST)
>>>     Pointer "&tmp32" points to an object whose effective type is "unsigned int" (32 bits, unsigned) but is dereferenced as a narrower "unsigned short" (16 bits, unsigned).  This may lead to unexpected results depending on machine endianness.
3096     		safe_unpack16((uint16_t *)&tmp32, buffer);
3097     	safe_unpack16(&msg_ptr->version, buffer);
3098     	safe_unpackstr_xmalloc(&msg_ptr->cluster_name, &tmp32, buffer);
3099
3100     	/* We find out the version of the caller right here so use
3101     	   that as the rpc_version. */

** CID 149285:  Null pointer dereferences  (FORWARD_NULL)
/slurm/src/common/slurm_persist_conn.c: 588 in slurm_persist_conn_open()

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*** CID 149285:  Null pointer dereferences  (FORWARD_NULL)
/slurm/src/common/slurm_persist_conn.c: 588 in slurm_persist_conn_open()
582     				error("%s: Failed to unpack persistent connection init resp message from %s:%d",
583     				      __func__,
584     				      persist_conn->rem_host,
585     				      persist_conn->rem_port);
586     			_close_fd(&persist_conn->fd);
587     		} else
>>>     CID 149285:  Null pointer dereferences  (FORWARD_NULL)
>>>     Dereferencing null pointer "resp".
588     			persist_conn->version = resp->ret_info;
589     	}
590
591     end_it:
592
593     	slurm_persist_free_rc_msg(resp);

** CID 149284:  Error handling issues  (CHECKED_RETURN)
/slurm/src/common/slurmdbd_defs.c: 1665 in _send_fini_msg()

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*** CID 149284:  Error handling issues  (CHECKED_RETURN)
/slurm/src/common/slurmdbd_defs.c: 1665 in _send_fini_msg()
1659     	buffer = init_buf(1024);
1660     	pack16((uint16_t) DBD_FINI, buffer);
1661     	req.commit  = 0;
1662     	req.close_conn   = 1;
1663     	slurmdbd_pack_fini_msg(&req, SLURM_PROTOCOL_VERSION, buffer);
1664
>>>     CID 149284:  Error handling issues  (CHECKED_RETURN)
>>>     Calling "slurm_persist_send_msg" without checking return value (as is done elsewhere 5 out of 6 times).
1665     	slurm_persist_send_msg(slurmdbd_conn, buffer);
1666     	free_buf(buffer);
1667
1668     	return SLURM_SUCCESS;
1669     }
1670