QmlJS: Fix potential crash in find usages.

When invalid documents or documents without idEnvironment or rootScope
get added to a QmlComponentChain.

Change-Id: I64b7062881f19777ba20efda6efa2e7bab02571e
Reviewed-by: Leandro Melo <leandro.melo@nokia.com>
(cherry picked from commit 9b411ed5)
Reviewed-by: Eike Ziller <eike.ziller@nokia.com>