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

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 <niallwalsh@users.berlios.de>
restart debSrcRecordParsers only if needed

The offset variable in DebSrcRecordParser was not initialized which we
now do and based on it do not trigger a restart if the parser was not
used yet avoiding a needless rescan of the section.

Detected while working on the previous commit e62aa1dd. Both commits act
as a "fix" for the bug shown in the testcase of the commit – this one
here would only hide it through.
fix various style/performance warnings in rred

Reported-By: cppcheck
Git-Dch: Ignore