src: operator[] checks bounds in debug mode

PR-URL: #16002
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

src/string_search.h

@@ -44,7 +44,9 @@ class Vector {
 
   // Access individual vector elements - checks bounds in debug mode.
   T& operator[](size_t index) const {
+#ifdef DEBUG
     CHECK(index < length_);
+#endif
     return start_[is_forward_ ? index : (length_ - index - 1)];
   }