Various CLANG fixes.

The error messages associated with these are:

/home/tim/slurm/src/common/callerid.c:337:21: warning: address of array 'entryp->d_name' will always evaluate to 'true' [-Wpointer-bool-conversion]
                else if (!entryp->d_name)
                         ~~~~~~~~~^~~~~~
1 warning generated.
/home/tim/slurm/src/common/gres.c:831:19: warning: comparison of unsigned expression < 0 is always false [-Wtautological-compare]
                if ((tmp_uint64 < 0) || (tmp_uint64 >= NO_VAL)) {
                     ~~~~~~~~~~ ^ ~
1 warning generated.
/home/tim/slurm/src/slurmctld/node_mgr.c:1684:26: warning: if statement has empty body [-Wempty-body]
                        if (first_new == NULL);
                                              ^
/home/tim/slurm/src/slurmctld/node_mgr.c:1684:26: note: put the semicolon on a separate line to silence this warning
/home/tim/slurm/src/slurmctld/node_mgr.c:1756:26: warning: if statement has empty body [-Wempty-body]
                        if (first_new == NULL);
                                              ^
/home/tim/slurm/src/slurmctld/node_mgr.c:1756:26: note: put the semicolon on a separate line to silence this warning
/home/tim/slurm/src/slurmctld/node_mgr.c:1830:26: warning: if statement has empty body [-Wempty-body]
                        if (first_new == NULL);
                                              ^
/home/tim/slurm/src/slurmctld/node_mgr.c:1830:26: note: put the semicolon on a separate line to silence this warning
/home/tim/slurm/src/plugins/switch/generic/switch_generic.c:837:27: warning: address of array 'if_rec->ifa_addr->sa_data' will always evaluate to 'true' [-Wpointer-bool-conversion]
                        if (!if_rec->ifa_addr->sa_data)
                            ~~~~~~~~~~~~~~~~~~~^~~~~~~