don't try to qsort() NULL array

while this actually works due to the array size being zero, it's
undefined behavior which makes gcc eliminate a subsequent null check in
the calling function.

src/main_sync.c

@@ -186,6 +186,7 @@ filter_boxes( string_list_t *boxes, const char *prefix, string_list_t *patterns
 			boxarr[num] = NULL;
 		}
 	}
+	if (boxarr)
 		qsort( boxarr, num, sizeof(*boxarr), cmp_box_names );
 	return boxarr;
 }