A small rbtree delete fixup optimization.

Setting node color to black is not required here because
it is already black.  Besides in the original algorithm
the node pointer is discarded and the node is set to tree
root just to quit the loop.

Thanks to 洪志道 (Hong Zhi Dao).

src/nxt_rbtree.c

@@ -399,7 +399,7 @@ nxt_rbtree_delete_fixup(nxt_rbtree_t *tree, nxt_rbtree_node_t *node)
 
             nxt_rbtree_left_rotate(parent);
 
-            break;
+            return;
 
         } else {
             sibling = parent->left;
@@ -437,7 +437,7 @@ nxt_rbtree_delete_fixup(nxt_rbtree_t *tree, nxt_rbtree_node_t *node)
 
             nxt_rbtree_right_rotate(parent);
 
-            break;
+            return;
         }
     }