analyzer: fix ICE copying struct [PR 94816]

analyzer: fix ICE copying struct [PR 94816]

PR analyzer/94816 reports an ICE when attempting to copy a struct
containing a field for which add_region_for_type for fails (on
an OFFSET_TYPE): the region for the src field comes from
make_region_for_unexpected_tree_code which gives it a NULL type, and
then the copy calls add_region_for_type which unconditionally
dereferences the NULL type.

This patch fixes the ICE by checking for NULL types in
add_region_for_type.

gcc/analyzer/ChangeLog:
	PR analyzer/94816
	* engine.cc (impl_region_model_context::on_unexpected_tree_code):
	Handle NULL tree.
	* region-model.cc (region_model::add_region_for_type): Handle
	NULL type.
	* region-model.h
	(test_region_model_context::on_unexpected_tree_code): Handle NULL
	tree.

gcc/testsuite/ChangeLog:
	PR analyzer/94816
	* g++.dg/analyzer/pr94816.C: New test.