restart debSrcRecordParsers only if needed

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.