re PR c++/10793 (ICE (treecheck) when partial-specializing with an invalid type and a base class, in xref_basetypes, at cp/decl.c:12834)

	PR c++/10793
	* decl.c (xref_basetypes): Handle error_mark_node.

	* g++.dg/template/crash9.C: New test.

From-SVN: r69671