PR c++/96440: ICE-on-invalid-code error recovery.

This patch fixes PR c++/96440 which is an ICE-on-invalid-code regression
affecting mainline.

2022-03-08  Roger Sayle  <roger@nextmovesoftware.com>

gcc/cp/ChangeLog
	PR c++/96440
	* decl.cc (start_decl): Defend against prefix_attributes being
	error_mark_node.

gcc/testsuite/ChangeLog
	PR c++/96440
	* g++.dg/cpp0x/pr96440.C: New test case.