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
Change-Id: Iab33cc7a8b79920350c72f054dff0e0a3cde69ce

libc/arch-arm64/generic/bionic/memchr.S

@@ -75,6 +75,7 @@ ENTRY(memchr)
 	 * Magic constant 0x40100401 allows us to identify which lane matches
 	 * the requested byte.
 	 */
+	cbz	cntin, .Lzero_length
 	mov	wtmp2, #0x0401
 	movk	wtmp2, #0x4010, lsl #16
 	dup	vrepchr.16b, chrin
@@ -157,4 +158,8 @@ ENTRY(memchr)
 	/* Select result or NULL */
 	csel	result, xzr, result, eq
 	ret
+
+.Lzero_length:
+	mov	result, xzr
+	ret
 END(memchr)