PR c/93640 - The write_only and read_write attributes can be mistyped due to invalid strncmp size argument

gcc/c-family/ChangeLog:

	PR c/93640
	* c-attribs.c (handle_access_attribute): Correct off-by-one mistakes.

gcc/testsuite/ChangeLog:

	PR c/93640
	* gcc.dg/attr-access.c: New test.