Fix bug in planner VaryMatchingStart iterator

Reviewers: teon.banek, florijan, buda

Reviewed By: teon.banek

Subscribers: pullbot

Differential Revision: https://phabricator.memgraph.io/D680

src/query/plan/variable_start_planner.cpp

@@ -208,7 +208,13 @@ class VaryMatchingStart {
       if (*start_nodes_it_ == self_.nodes_.end()) {
         return *this;
       }
-      const auto &start_node = *(*start_nodes_it_)++;
+      ++*start_nodes_it_;
+      // start_nodes_it_ can become equal to `end` and we shouldn't dereference
+      // iterator in that case.
+      if (*start_nodes_it_ == self_.nodes_.end()) {
+        return *this;
+      }
+      const auto &start_node = **start_nodes_it_;
       current_matching_ =
           Matching{ExpansionsFrom(start_node, self_.matching_.expansions,
                                   self_.symbol_table_),