Creating a new Servercheck Extension without a "shortName" causes segfault

[ocket8888]

This Bug Report affects these Traffic Control components:

• Traffic Ops

Current behavior:

Creating a new Servercheck without a servercheck_shortname property (or one set to null) causes a nil pointer dereference segmentation fault and returns a 500 Internal Server Error response.

Expected behavior:

If a required property is missing from a request, the response should be a 400 Bad Request with an error-level Alert describing what's missing.

Steps to reproduce:

1. topost -kfp servercheck/extensions '{"name": "", "info_url": "", "version": "", "script_file": "", "isactive": 0, "type": "CHECK_EXTENSION_BOOL"}'