Handle errors in both args of va_arg

2016-08-29  Tom de Vries  <tom@codesourcery.com>

	PR c/77398
	* c-common.c (build_va_arg): Add first argument error.  Build va_arg
	with error_mark_node as va_list instead of with illegal va_list.

	* gimplify.c (gimplify_va_arg_expr): Replace first argument type error
	with assert.

	* g++.dg/ext/va-arg1.C: Add error check for illegal first argument.

From-SVN: r239827