CppTools: Remove a QTC_ASSERT

Triggering the action "Rename Symbol Under Cursor" before the
SnapshotUpdater::update() call in parse() of cpptoolseditorsupport.cpp
led to the failing assertion.

Triggering the action in the initialization phase of the editor is not
supported, therefore replace the assert with a silent if.

Change-Id: Ib911b8aa038ae3d9ea28c720853780b11dfa0fb7
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>