tree-optimization/107554 - fix ICE in stlen optimization

The following fixes a wrongly typed variable causing an ICE.

	PR tree-optimization/107554
	* tree-ssa-strlen.cc (strlen_pass::count_nonzero_bytes):
	Use unsigned HOST_WIDE_INT type for the strlen.

	* gcc.dg/pr107554.c: New testcase.

Co-Authored-By: Nikita Voronov <nik_1357@mail.ru>