malloc: Check for integer overflow in valloc.

A large bytes parameter to valloc could cause an integer overflow
and corrupt allocator internals. Check the overflow does not occur
before continuing with the allocation.

ChangeLog:

2013-09-11  Will Newton  <will.newton@linaro.org>

	[BZ #15856]
	* malloc/malloc.c (__libc_valloc): Check the value of bytes
	does not overflow.