mxq_job: Fix invalid pointer deref on error path

[donald]

I've got this from valgrind:

2020-04-15 16:11:00 +0200 mxqd[6617]: mxqd mxqd.c:2600:main(): DEBUG: group_cnt=36 :: 36 Groups loaded
2020-04-15 16:11:00 +0200 mxqd[6617]: mxqd mxqd.c:1343:start_user(): DEBUG:  user=buczek(125) slots_to_start=11 :: trying to start jobs for user.
2020-04-15 16:11:00 +0200 mxqd[6617]: mxqd mxqd.c:1370:start_user():   group=buczek(125):335148 slots_to_start=11 slots_per_job=11 :: trying to start job for group.
==6617== Conditional jump or move depends on uninitialised value(s)
==6617==    at 0x40318FB: free (vg_replace_malloc.c:540)
==6617==    by 0x415307: mxq_select_job_from_group (mxq_job.c:324)
==6617==    by 0x41545F: mxq_assign_job_from_group_to_daemon (mxq_job.c:355)
==6617==    by 0x416849: mxq_load_job_from_group_for_daemon (mxq_job.c:740)
==6617==    by 0x40715F: start_job (mxqd.c:1188)
==6617==    by 0x407B82: start_user (mxqd.c:1372)
==6617==    by 0x407CC1: start_user_with_least_running_global_slot_count (mxqd.c:1412)
==6617==    by 0x40B450: main (mxqd.c:2609)
==6617==
2020-04-15 16:11:00 +0200 mxqd[6617]: mxqd mxq_job.c:746:mxq_load_job_from_group_for_daemon(): WARNING:   group_id=335148 :: mxq_assign_job_from_group_to_daemon(): No matching job found - maybe another server was a bit faster. ;)

The pointer freed at the end of mxq_select_job_from_group() is supposed
to be allocated by mx_mysql_do_statement(). However, the pointer is
only allocated in the common case, where the routine delivers a result:

if (result && result->count && num_rows) {
    tmpdata = mx_calloc_forever(num_rows, size);
    ...
    *to = tmpdata;

In the unusual case that there was no result from the query (as above,
because other servers already started all pending jobs) or in error
cases, job_id_out might be uninitialized and the free() might
is invalid.

Make sure, we don't try to free via an unitialized pointer. Do not decide
based on the return value of mx_mysql_do_statement(), because we don't
want to duplicate the complicated logic in the if stated quited above to
be duplicated in the caller.