Fix UBSan breakage due to pointer/ref issue

Bug: v8:9241
Change-Id: I0e2b73e3f98b1863e790a7d28a716fe9e3478103
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1610437Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Frank Tang <ftang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61455}

src/objects/js-number-format.cc

@@ -728,7 +728,9 @@ Maybe<icu::UnicodeString> IcuFormatNumber(
     double number = numeric_obj->Number();
     formatted = number_format.formatDouble(number, status);
   }
-  formatted.getAllFieldPositions(*fp_iter, status);
+  if (fp_iter) {
+    formatted.getAllFieldPositions(*fp_iter, status);
+  }
   icu::UnicodeString result = formatted.toString(status);
   if (U_FAILURE(status)) {
     THROW_NEW_ERROR_RETURN_VALUE(isolate,