Handle cleanly the case when pull["head"]["repo"] returns None.

This may be linked to the user sending a PR from his master branch.
For example, see https://github.com/sympy/sympy/pull/1793
Merge pull request #148 from jrioux/reviews

Handle cleanly the case when pull["head"]["repo"] returns None.

This may be linked to the user sending a PR from his master branch.
For example, see sympy/sympy#1793