[regexp] Fix invalid access into empty string

If `out` is empty accessing `out.back()` is invalid.

TBR=yangguo@chromium.org

Bug: chromium:894934
Change-Id: I7286c5b6a9857f1cdb2bcaf383094bee65bac393
Reviewed-on: https://chromium-review.googlesource.com/c/1282565Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56669}

test/fuzzer/regexp-builtins.cc

@@ -103,7 +103,7 @@ std::string NaiveEscape(const std::string& input, char escaped_char) {
   }
   // Disallow trailing backslashes as they mess with our naive source string
   // concatenation.
-  if (out.back() == '\\') out.back() = '_';
+  if (!out.empty() && out.back() == '\\') out.back() = '_';
 
   return out;
 }