Fix unknown RPC crash

[joeljfischer]

Fixes #1713

This PR is ready for review.

Risk

This PR makes no API changes.

Testing Plan

• I have verified that I have not introduced new warnings in this PR (or explain why below)
• I have run the unit tests with this PR
• I have tested this PR against Core and verified behavior (if applicable, if not applicable, explain why below).

Unit Tests

No unit test changes

Core Tests

Tested receiving the errant RPC type from Sync 3.4

Core version / branch / commit hash / module tested against: Ford Sync 3.4
HMI name / version / branch / commit hash / module tested against: Ford Sync 3.4

Summary

If a received protocol message cannot be turned into an RPC, discard it.

Changelog

Bug Fixes

• When an unknown RPC is received, discard it instead of crashing.

CLA

• I have signed the CLA

[codecov]

Codecov Report

Merging #1714 into develop will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop    #1714      +/-   ##
===========================================
+ Coverage    83.79%   83.81%   +0.02%     
===========================================
  Files          407      407              
  Lines        20811    20813       +2     
===========================================
+ Hits         17438    17444       +6     
+ Misses        3373     3369       -4