get group again after potential remap in Source: parse

get group again after potential remap in Source: parse

Mysteriously segfaults only on i386 for me, but at least one reporter
had the same behavior and it makes sense that this is the problem as the
parsing of Source: was fixed in 1.2.2 – before the not remapped group
was not used.

We don't use our usual Dynamic<> trick here as we don't have it in the
parser. Its a bit of a layer violation to do this parsing here, but its
how it is always was…

Until next time with this lovely kind of problem.

Closes: 812251
Thanks: Francesco Poli and Marc Haber for testdata.