Check that message code exists before accessing

[jacobkeeler]

Fixes #3846

This PR is ready for review.

Risk

This PR makes no API changes.

Testing Plan

• Run ATF test test_scripts/Policies/build_options/070_ATF_PTU_Merge_Into_Local_PT_ConsumerFriendlyMessages_Omitted_HTTP.lua with HTTP policies

Summary

The GetUserFriendlyMsg function was inadvertently creating an empty messages entry when searching for a message code which didn't exist in the policy table during the PTU process, causing the table to be corrupted. This PR adds a check to this function to prevent this from happening.

Changelog

Bug Fixes

• Fix accidental creation of empty messages entry in GetUserFriendlyMsg

CLA

• [x] I have signed the CLA

[jacobkeeler]

Note that I was only able to reproduce the issue directly in HTTP policy mode, but the check was added to all policy modes for safety