gsignal: add assert on closure invalidate path

It's theoretically possible that we could have a case where this would
actually return NULL, but it's difficult to imagine a valid program that
would contain such a case.

Add an explicit assert here to quiet up static analysis.

See the bug for more discussion.

Coverity CID: 1159477

https://bugzilla.gnome.org/show_bug.cgi?id=730296