gcc/ChangeLog:

	PR middle-end/91977
	* tree-ssa-strlen.c (count_nonzero_bytes): Handle assignments with
	MEM_REF right operand.  Avoid failing for MEM_REF assignments from
	uninitialized objects.

gcc/testsuite/ChangeLog:

	PR middle-end/91977
	* gcc.dg/Wstringop-overflow-18.c: New test.

From-SVN: r276603