analyzer: fix ICE when DECL_INITIAL is error_mark_node [PR98580]

analyzer: fix ICE when DECL_INITIAL is error_mark_node [PR98580]

lto-streamer-out.c's get_symbol_initial_value can return error_mark_node
rather than DECL_INITIAL as an optimization to avoid extra sections for
simple scalar values.

Add a check to the analyzer to handle such cases gracefully.

gcc/analyzer/ChangeLog:
	PR analyzer/98580
	* region.cc (decl_region::get_svalue_for_initializer): Gracefully
	handle when LTO writes out DECL_INITIAL as error_mark_node.

gcc/testsuite/ChangeLog:
	PR analyzer/98580
	* gcc.dg/analyzer/pr98580-a.c: New test.
	* gcc.dg/analyzer/pr98580-b.c: New test.