[ObjC] Ensure -[GPBCodedOutputStream dealloc] cannot throw exceptions

If `-[GPBCodedOutputStream flush]` failed (e.g., because the filesystem
was out of space), then `-[GPBCodedOutputStream dealloc]` would throw
an exception.

`-dealloc` cannot fail, so the only thing to do in this case is to silently
swallow the exception.

PiperOrigin-RevId: 579916429