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

tests/string_test.cpp

@@ -763,6 +763,14 @@ TEST(string, memchr) {
   }
 }
 
+TEST(string, memchr_zero) {
+  uint8_t* buffer;
+  ASSERT_EQ(0, posix_memalign(reinterpret_cast<void**>(&buffer), 64, 64));
+  memset(buffer, 10, 64);
+  ASSERT_TRUE(NULL == memchr(buffer, 5, 0));
+  ASSERT_TRUE(NULL == memchr(buffer, 10, 0));
+}
+
 TEST(string, memrchr) {
   int seek_char = random() & 255;
   StringTestState<char> state(SMALL);