Fix stale char_ptr for find_first_character_wide read

On exit from the loop, char_ptr had not been updated to match
block_ptr, resulting in erroneous results. Moving all updates out of
the loop fixes that.

Author: Sterling-Augustine

libc/src/string/string_utils.h

@@ -136,11 +136,11 @@ find_first_character_wide_read(const unsigned char *src, unsigned char ch,
   const Word ch_mask = repeat_byte<Word>(ch);
 
   // Step 2: read blocks
-  for (const Word *block_ptr = reinterpret_cast<const Word *>(char_ptr);
-       !has_zeroes<Word>((*block_ptr) ^ ch_mask) && cur < n;
-       ++block_ptr, cur += sizeof(Word)) {
-    char_ptr = reinterpret_cast<const unsigned char *>(block_ptr);
-  }
+  const Word *block_ptr = reinterpret_cast<const Word *>(char_ptr);
+  for (; !has_zeroes<Word>((*block_ptr) ^ ch_mask) && cur < n;
+       ++block_ptr, cur += sizeof(Word))
+    ;
+  char_ptr = reinterpret_cast<const unsigned char *>(block_ptr);
 
   // Step 3: find the match in the block
   for (; *char_ptr != ch && cur < n; ++char_ptr, ++cur) {

libc/test/src/string/memchr_test.cpp

@@ -21,6 +21,11 @@ const char *call_memchr(const void *src, int c, size_t size) {
   return reinterpret_cast<const char *>(LIBC_NAMESPACE::memchr(src, c, size));
 }
 
+TEST(LlvmLibcMemChrTest, FromProtoC) {
+  const char *src = "protobuf_cpp_version$\n";
+  ASSERT_STREQ(call_memchr(src, '$', 22), "$\n");
+}
+
 TEST(LlvmLibcMemChrTest, FindsCharacterAfterNullTerminator) {
   // memchr should continue searching after a null terminator.
   const size_t size = 5;