Don't crash on Runtime::getQmlImportedScripts

Task-number: QTBUG-37303

Change-Id: I083ca0cc3223fe0dbf1c768ad43a0d4927579867

Conflicts:
	tests/auto/qml/qqmlecmascript/tst_qqmlecmascript.cpp
Delete objects created by qmlplugindump

This helps plugin objects to shut down cleanly