Fix two memory leaks (#3747)

* Fix two memory leaks

core/block_dragger.js

@@ -291,6 +291,7 @@ Blockly.BlockDragger.prototype.maybeDeleteBlock_ = function() {
     // Fire a move event, so we know where to go back to for an undo.
     this.fireMoveEvent_();
     this.draggingBlock_.dispose(false, true);
+    Blockly.draggingConnections = [];
   } else if (trashcan) {
     // Make sure the trash can is closed.
     trashcan.close();