c++, coroutines: Check for malformed functions before splitting.

This performs the same basic check that is done by finish_function
to catch cases where the function is so badly malformed that we
do not have a consistent binding level.

gcc/cp/ChangeLog:

	* coroutines.cc (split_coroutine_body_from_ramp): Check
	that the binding level is as expected before attempting
	to outline the function body.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>