Fix crash in MLD alternative search if source or target are invalid (#5851)

In situations where there is not a valid source or target phantom
node (e.g. when snapping to an edge with a zero weight), a
heap assertion will fail in the MLD alternative search code.

We fix this by checking for empty heaps before proceeding with
the search.

src/engine/routing_algorithms/alternative_path_mld.cpp

@@ -663,6 +663,10 @@ makeCandidateVias(SearchEngineData<Algorithm> &search_engine_data,
     Heap &reverse_heap = *search_engine_data.reverse_heap_1;
 
     insertNodesInHeaps(forward_heap, reverse_heap, phantom_node_pair);
+    if (forward_heap.Empty() || reverse_heap.Empty())
+    {
+        return {};
+    }
 
     // The single via node in the shortest paths s,via and via,t sub-paths and
     // the weight for the shortest path s,t we return and compare alternatives to.