Fix memchr with a zero length.

The memchr implementation for 64 bit fails if these conditions occur:

- The buffer is 32 byte aligned.
- The buffer contains the character in the first byte.
- The count sent in is zero.

The function should return NULL, but it's not.

Bug: 16676625

(cherry picked from commit e03e1eac)

Change-Id: Ie4cca2c445127a0936ee2b96651a8e7204fbaffd