alias.c (find_base_term, get_addr): Do not dereference NULL pointer when all VALUE's locations has been invalidated.

	* alias.c (find_base_term, get_addr):  Do not dereference NULL
	pointer when all VALUE's locations has been invalidated.
	(rtx_equal_for_memref_p): Simplify checking of VALUEs.

From-SVN: r77201