Fix field_index in error message for validation of message header

Bug: 778067
Change-Id: Ib61c658583e4f89ed94af229ded597baa4ee1887
Reviewed-on: https://chromium-review.googlesource.com/744341Reviewed-by: Yuzhu Shen <yzshen@chromium.org>
Commit-Queue: agrieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512600}

mojo/public/cpp/bindings/lib/message_header_validator.cc

@@ -73,7 +73,7 @@ bool IsValidMessageHeader(const internal::MessageHeader* header,
   //   payload size).
   // - Validation of the payload contents will be done separately based on the
   //   payload type.
-  if (!internal::ValidatePointerNonNullable(header_v2->payload, 0,
+  if (!internal::ValidatePointerNonNullable(header_v2->payload, 5,
                                             validation_context) ||
       !internal::ValidatePointer(header_v2->payload, validation_context) ||
       !validation_context->ClaimMemory(header_v2->payload.Get(), 1)) {