Avoid critical error in thread check if source is destroyed

Avoid critical error in thread check if source is destroyed

The source can be destroyed by the time we complete the result, and
then the g_source_get_context(current_source) call will cause
a critical error. We check for the source being destroyed and avoid
the check in that case.

This means we miss the right-thread check in this case, but thats
merely a helper, so this is not critical.