Fixed erase return iterator.

Author: 5cript

include/interval-tree/interval_tree.hpp

@@ -885,14 +885,20 @@ namespace lib_interval_tree
             if (!iter.node_)
                 throw std::out_of_range("cannot erase end iterator");
 
-            auto next = iter;
-            ++next;
+            iterator next = iter;
 
             node_type* y;
             if (!iter.node_->left_ || !iter.node_->right_)
+            {
                 y = iter.node_;
+                ++next;
+            }
             else
+            {
                 y = successor(iter.node_);
+                next = iterator{y, this};
+                ++next;
+            }
 
             node_type* x;
             if (y->left_)