Add missing store in emission of asan_stack_free.

gcc/ChangeLog:

2020-05-19  Martin Liska  <mliska@suse.cz>

	PR sanitizer/94910
	* asan.c (asan_emit_stack_protection): Emit
	also **SavedFlagPtr(FakeStack, class_id) = 0 in order to release
	a stack frame.