Follow-up to bdb8c3729f

Fixing ExtrusionEntityCollection append move operator
(fixing double destruction of ExtrusionEntityCollections)

src/libslic3r/ExtrusionEntityCollection.hpp

@@ -68,6 +68,9 @@ public:
             entities.insert(entities.end(),
                 std::make_move_iterator(src.begin()),
                 std::make_move_iterator(src.end()));
+            // Removing pointers to polymorphic extrusions from the donor object
+            // so that they will not be deleted twice.
+            src.clear();
         }
     }
     void append(const ExtrusionPaths &paths) {