Overview
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The Github repository permissions feature of the Github Commiter Authorization Strategy in the github-oauth-plugin throws a fatal error when the Github URL for the Job is an invalid URL (e.g., 404). The authorization strategy should fail gracefully when the repo URL is invalid.

Steps To Reproduce
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1) Configure Jenkins Global security (http://jenkins/configureSecurity) to use the Github Authentication Plugin and use the "Github Commiter Authorization Strategy" with the option enabled for "Use Github repository permissions"
2) Create a new Job with a Git SCM provider (e.g., "MyJob")
3) Enter an invalid Git Repository URL that does not exist (e.g., git@github.com:MyOrg/wrong-repo-name.git instead of git@github.com:MyOrg/correct-repo-name.git)
4) Save the Job config (note: ignore the warnings about an invalid Git URL; we've seen valid URLs renamed, moved to new orgs, etc)

Actual Result
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The Jenkins web UI URL for the Job (e.g., http://jenkins/job/MyJob) responds with a 500 error caused by a NullReferenceException in the Github authorization strategy

Expected Result
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The Jenkins web UI URL for the job should respond with a 200 and render the normal view for the job. Attempts to build the job should fail due to the invalid Git URL.