c++: Fix error recovery for invalid arrays [PR123331]

c++: Fix error recovery for invalid arrays [PR123331]

The following testcase ICEs since the conditional
eltinit = build2 (INIT_EXPR, ...) has been added to cxx_eval_vec_init_1.

If there are errors, eltinit will be error_mark_node and we can ICE during
constant evaluation of that.

Fixed by skipping it for error operands.

2026-01-03  Jakub Jelinek  <jakub@redhat.com>

	PR c++/123331
	* constexpr.cc (cxx_eval_vec_init_1): Don't build INIT_EXPR if
	eltinit is erroneous.

	* g++.dg/other/pr123331.C: New test.