The snapshot was given as a temporary expression to
findMatchingDefinition(). Upon destruction it deleted documents/controls
to which the returned symbols still had pointers to. The prime candidate
for the document to free is the one coming from the editor - the user
typed new content and thus created a new document. The last reference to
the old document was held in the temporary argument expression.

Fix by pinning the snapshot.

Task-number: QTCREATORBUG-16062
Change-Id: I4fb3915bef8471cefef4ca27450901824416e397
Reviewed-by: Orgad Shaneh <orgads@gmail.com>