middle-end/108995 - avoid folding when sanitizing overflow

The following plugs one place in extract_muldiv where it should avoid
folding when sanitizing overflow.

	PR middle-end/108995
	* fold-const.cc (extract_muldiv_1): Avoid folding
	(CST * b) / CST2 when sanitizing overflow and we rely on
	overflow being undefined.

	* gcc.dg/ubsan/pr108995.c: New testcase.