jobcomp/elasticsearch - fix memory leak when transferring generated buffer.

Running slurmctld under valgrind while operating with jobcomp/elasticsearch
reported the following bytes definitely lost:

==27403== 658 bytes in 1 blocks are definitely lost in loss record 301 of 342
==27403==    at 0x4C2FD4F: realloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==27403==    by 0x2281B3: slurm_xrealloc (xmalloc.c:137)
==27403==    by 0x22856A: makespace (xstring.c:114)
==27403==    by 0x2285D0: _xstrcat (xstring.c:132)
==27403==    by 0x228CE0: _xstrfmtcat (xstring.c:291)
==27403==    by 0x83C5BCD: ???
==27403==    by 0x30A913: g_slurm_jobcomp_write (slurm_jobcomp.c:172)
==27403==    by 0x18D8FC: job_completion_logger (job_mgr.c:13652)

It turns out the generated buffer in slurm_jobcomp_log_record was xstrdup'ed to
the corresponding job_node->serialized_job, but the originally generated buffer
wasn't freed afterwards. The fix consists in change the transfer so that instead
of xstrdup'ing the char * we just assign the pointer and NULL the buffer.

The job_node->serialized_job was already xfree'd properly later when the job
was indexed.

Discovered while working on Bug 4065.