Reset existingRefs on cancel

When placing, we need to keep our static pointer map valid.  If we
cancel the placement after annotation, we need to remove the annotated
symbol.  The safer way is to just recreate the map

Fixes https://gitlab.com/kicad/code/kicad/issues/12983

eeschema/tools/sch_drawing_tools.cpp

@@ -172,6 +172,9 @@ int SCH_DRAWING_TOOLS::PlaceSymbol( const TOOL_EVENT& aEvent )
             {
                 m_toolMgr->RunAction( EE_ACTIONS::clearSelection, true );
                 m_frame->RollbackSchematicFromUndo();
+                existingRefs.Clear();
+                hierarchy.GetSymbols( existingRefs );
+                existingRefs.SortByReferenceOnly();
                 symbol = nullptr;
             };