Fixed unit library mutex usage.

For the optimization purpose, function nxt_unit_remove_process() expects
lib->mutex to be locked.  The function then moves ports queue into temporary
queue and releases mutex.  In nxt_unit_done() there were two errors: mutex was
not locked before nxt_unit_remove_process() call and mutex was not destroyed.
It is hard to tell what was possible negative impact of this errors.

Found by Coverity (CID 308517).

src/nxt_unit.c

@@ -2728,14 +2728,20 @@ nxt_unit_done(nxt_unit_ctx_t *ctx)
     } nxt_queue_loop;
 
     for ( ;; ) {
+        pthread_mutex_lock(&lib->mutex);
+
         process = nxt_unit_process_pop_first(lib);
         if (process == NULL) {
+            pthread_mutex_unlock(&lib->mutex);
+
             break;
         }
 
         nxt_unit_remove_process(ctx, process);
     }
 
+    pthread_mutex_destroy(&lib->mutex);
+
     free(lib);
 }