branches/zip: buf_flush_try_neighbors(): Fix a bug

that was introduced in r3879 (rb://73).

buf/buf0flu.c

@@ -918,9 +918,9 @@ buf_flush_try_neighbors(
 				count++;
 
 				buf_pool_mutex_enter();
+			} else {
+				mutex_exit(block_mutex);
 			}
-
-			mutex_exit(block_mutex);
 		}
 	}