buf: Is this the function you were looking for?

src/buf_text.c

@@ -111,7 +111,7 @@ bool git_buf_text_is_binary(const git_buf *buf)
 
 bool git_buf_text_contains_nul(const git_buf *buf)
 {
-	return (strnlen(buf->ptr, buf->size) != buf->size);
+	return (memchr(buf->ptr, '\0', buf->size) != NULL);
 }
 
 int git_buf_text_detect_bom(git_bom_t *bom, const git_buf *buf, size_t offset)