malloc/hooks.c: Correct check for overflow in memalign_check.

malloc/hooks.c: Correct check for overflow in memalign_check.

A large value of bytes passed to memalign_check can cause an integer
overflow in _int_memalign and heap corruption. This issue can be
exposed by running tst-memalign with MALLOC_CHECK_=3.

ChangeLog:

2013-10-10  Will Newton  <will.newton@linaro.org>

	* malloc/hooks.c (memalign_check): Ensure the value of bytes
	passed to _int_memalign does not overflow.