Avoiding arithmetic ops with NULL pointer in nxt_unit_mmap_get

Found by UndefinedBehaviorSanitizer.

Reviewed-by: Andrew Clayton <a.clayton@nginx.com>

Author: andrey-zelenkov

src/nxt_unit.c

@@ -3502,6 +3502,10 @@ nxt_unit_mmap_get(nxt_unit_ctx_t *ctx, nxt_unit_port_t *port,
 
     pthread_mutex_lock(&lib->outgoing.mutex);
 
+    if (nxt_slow_path(lib->outgoing.elts == NULL)) {
+        goto skip;
+    }
+
 retry:
 
     outgoing_size = lib->outgoing.size;
@@ -3598,6 +3602,8 @@ nxt_unit_mmap_get(nxt_unit_ctx_t *ctx, nxt_unit_port_t *port,
         goto retry;
     }
 
+skip:
+
     *c = 0;
     hdr = nxt_unit_new_mmap(ctx, port, *n);