tap: do not call output before int::output_tap is set

A call into output before int::output_tap is set will result in a
stacktrace since output depends on int::output_tap to decide output
format.

Signed-off-by: Niklas Söderlund <niso@kth.se>
Merge pull request #3 from neg/master

tap: do not call output before int::output_tap is set