patch 7.4.1814

Problem:    A channel may be garbage collected while it's still being used by
            a job. (James McCoy)
Solution:   Mark the channel as used if the job is still used.  Do the same
            for channels that are still used.

src/eval.c

@@ -7024,6 +7024,7 @@ garbage_collect(int testing)
 
 #ifdef FEAT_JOB_CHANNEL
     abort = abort || set_ref_in_channel(copyID);
+    abort = abort || set_ref_in_job(copyID);
 #endif
 #ifdef FEAT_NETBEANS_INTG
     abort = abort || set_ref_in_nb_channel(copyID);