re PR c/70651 (ICE on invalid code on x86_64-linux-gnu in build_va_arg, at c-family/c-common.c:5728)

	PR c/70651
	* c-common.c (build_va_arg): Change two asserts into errors and return
	error_mark_node.

	* c-c++-common/pr70651.c: New test.

From-SVN: r235027