While reducing something else I noticed that we ICE on the following
invalid code.  In tsubst_lambda_expr, tsubst_template_decl has already
reported an error and returned the error_mark_node, so make sure we
don't ICE on that.  I'm using a goto here because we still have to
do finish_struct because it does popclass ().

	PR c++/94507 - ICE-on-invalid with lambda template.
	* pt.c (tsubst_lambda_expr): Cope when tsubst_template_decl or
	tsubst_function_decl returns error_mark_node.

	* g++.dg/cpp2a/lambda-generic7.C: New test.