some fixes

Author: tzaeschke

include/phtree/common/b_plus_tree_multimap.h

@@ -419,9 +419,10 @@ class b_plus_tree_multimap {
             if (data_.empty()) {
                 // Nothing to merge, just remove node. This should be rare, i.e. only happens when
                 // a rare 1-entry node has its last entry removed.
+                auto next_node = next_node_;  // create copy because (this) will be deleted
                 remove_from_siblings();
                 parent_->remove_node(max_key_old, this, tree);
-                return next_node_ == nullptr ? ER{} : ER{next_node_, next_node_->data_.begin()};
+                return next_node == nullptr ? ER{} : ER{next_node, next_node->data_.begin()};
             }
 
             if (data_.size() < this->M_min()) {
@@ -500,9 +501,10 @@ class b_plus_tree_multimap {
             if (data_.empty()) {
                 // Nothing to merge, just remove node. This should be rare, i.e. only happens when
                 // a rare 1-entry node has its last entry removed.
+                auto next_node = next_node_;  // create copy because (this) will be deleted
                 remove_from_siblings();
                 parent_->remove_node(max_key_old, this, tree);
-                return next_node_ == nullptr ? ER{} : ER{next_node_, next_node_->data_.begin()};
+                return next_node == nullptr ? ER{} : ER{next_node, next_node->data_.begin()};
             }
 
             if (data_.size() < this->M_min()) {