c++: Fix up sb as condition instantiation error-recovery [PR120039]

The following two testcases ICE during instantation.
Normally for the artificial var used for structured binding as condition
(which has NULL DECL_NAME) there is a DECL_EXPR which registers local
specialization and so tsubst_expr works fine.
But if there are errors while parsing the initializer, the VAR_DECL
has still NULL_TREE DECL_NAME, but error_mark_node TREE_TYPE and
when tsubst_expr is called on it, it falls back to calling lookup_name
(NULL_TREE) and ICEs on that.

The following patch fixes it by not calling lookup_name if DECL_NAME is
NULL.

2026-03-16  Jakub Jelinek  <jakub@redhat.com>

	PR c++/120039
	PR c++/122559
	* pt.cc (tsubst_expr) <case VAR_DECL>: Don't call lookup_name on
	DECL_NAME (t) if it is NULL_TREE.

	* g++.dg/cpp26/decomp28.C: New test.
	* g++.dg/cpp26/decomp29.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>
(cherry picked from commit caae42b31118f754fbd1f9566373f589d7e1568f)