CppEditor: Fix crash in CppCodeModelInspectorDialog

We assumed there is always a BuiltinEditorDocumentParser::get() object,
but that's not the case if clang is used.

Task-number: QTCREATORBUG-13306
Change-Id: Ia48f95a32bf8fd1d208c2a5e70a1af41193d1fd1
Reviewed-by: Christian Stenger <christian.stenger@digia.com>