Check for nil http response.

[cjyar]

The nil resp can happen, for example, if this program sends an invalid auth token to GitHub. If it happens, it results in a panic which is caught by the http library.

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 187

• 7 of 7 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 184:	0.0%
Covered Lines:	414
Relevant Lines:	414

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💛 - Coveralls