pkgTagFile: if we have seen the end, do not try to see more

2014-01-29T23:16:19Z

Asking for more via Step() will notice that we are done with the file already and will result in a fail, which means we can't find the last sections anymore (which is especially painful if we haven't moved at all as in the testcase we haven't even looked at one of the sources leading to a strange behaviour) Reported-By: Niall Walsh

apt/test/integration/test-apt-get-source-multisources, branch 1.1.exp1

pkgTagFile: if we have seen the end, do not try to see more