fix tdelete to properly balance the tree

the tsearch data structure is an avl tree, but it did not implement
the deletion operation correctly so the tree could become unbalanced.

reported by Ed Schouten.

src/search/tsearch_avl.c

@@ -105,10 +105,13 @@ static struct node *insert(struct node **n, const void *k,
 	return r;
 }
 
-static struct node *movr(struct node *n, struct node *r) {
-	if (!n)
-		return r;
-	n->right = movr(n->right, r);
+static struct node *remove_rightmost(struct node *n, struct node **rightmost)
+{
+	if (!n->right) {
+		*rightmost = n;
+		return n->left;
+	}
+	n->right = remove_rightmost(n->right, rightmost);
 	return balance(n);
 }
 
@@ -122,7 +125,13 @@ static struct node *remove(struct node **n, const void *k,
 	c = cmp(k, (*n)->key);
 	if (c == 0) {
 		struct node *r = *n;
-		*n = movr(r->left, r->right);
+		if (r->left) {
+			r->left = remove_rightmost(r->left, n);
+			(*n)->left = r->left;
+			(*n)->right = r->right;
+			*n = balance(*n);
+		} else
+			*n = r->right;
 		free(r);
 		return parent;
 	}