posix/aio_misc-c/rtems_aio_completed_list_op(): Path with no lock release.

Coverity CID 1619144

In rtems_aio_completed_list_op() memory was getting freed without releasing
a lock.

This commit should also solve coverity issues 1619145, 1619143 and 1619142
since they all seem to be generated by rtems_aio_completed_list_op().

Closes #5127

cpukit/posix/src/aio_misc.c

@@ -266,6 +266,7 @@ void rtems_aio_completed_list_op( listcb *listcbp )
         );
         break;
     }
+    pthread_mutex_unlock( &listcbp->mutex );
     free( listcbp );
   } else {
     pthread_mutex_unlock( &listcbp->mutex );