PR tree-optimization/87490 - ICE in expand_builtin_strnlen with a constant argument and non-constant bound

gcc/ChangeLog:

	PR tree-optimization/87490
	* builtins.c (expand_builtin_strnlen): Handle a null data.decl
	consistently.

gcc/testsuite/ChangeLog:

	PR tree-optimization/87490
	* gcc.dg/pr87490.c: New test.
	* gcc.dg/warn-strnlen-no-nul-2.c: Same.

From-SVN: r264876