Fix memory leak in balancing.

src/main.cc

@@ -21,18 +21,19 @@ int main() {
   vase.insert(100, "gr\ntt", 5);
   vase.insert(20, "gr\ntt", 5);
 
-  vase.type(5, 'k');
-  vase.type(6, 'l');
-  vase.type(7, 'm');
-  vase.type(8, 'n');
+  vase.type(5, '1');
+  vase.type(6, '2');
+  vase.type(7, '3');
+  vase.type(8, '4');
+
+  vase.erase(9, -2);
 
   print_shard(vase.root);
 
   LineIterator it(vase.root, 3);
   std::string line;
-  while (it.next(line)) {
+  while (it.next(line))
     std::cout << line << "\n";
-  }
 
   std::cout << "\n\n"
             << vase.to_string();

src/vase/shard.cc

@@ -43,10 +43,12 @@ Shard *balance(Shard *node) {
   if (bf > 1) {
     Branch *left = (Branch *)b->left;
     if (balance_factor(left) < 0) {
+      Shard::retain(left);
       auto new_left = rotate_left(left);
       auto rebuilt = new Branch(new_left, b->right);
       auto result = rotate_right((Branch *)rebuilt);
       Shard::release(new_left);
+      Shard::release(b);
       return result;
     }
     return rotate_right(b);
@@ -55,10 +57,12 @@ Shard *balance(Shard *node) {
   if (bf < -1) {
     Branch *right = (Branch *)b->right;
     if (balance_factor(right) > 0) {
+      Shard::retain(right);
       auto new_right = rotate_right(right);
       auto rebuilt = new Branch(b->left, new_right);
       auto result = rotate_left((Branch *)rebuilt);
       Shard::release(new_right);
+      Shard::release(b);
       return result;
     }
     return rotate_left(b);