Prevent possible crash on 0 instant answers

This crash in `[UVBaseInstantAnswersViewController selectInstantAnswerAtIndex:]` was reported via Crashlytics:

    *** -[__NSArrayI objectAtIndex:]: index 0 beyond bounds for empty array

Seemingly there were no instant answers in the array, this little change checks for that. Feel free to cherry-pick because I have two other changes or ignore this pull request and implement the check yourself!