Perl_sv_vcatpvfn_flags: move %*v handling earlier

Perl_sv_vcatpvfn_flags: move %*v handling earlier

Where the v flag appears, and it has non-default separator, i.e.
*v or *NNN$v, retrieve the next or NNNth arg (which defines the separator)
earlier - as soon as we encounter the v flag. This should in theory make
no functional difference since no args are processed between those two
points (so no chance of us stealing something else's arg).

Doing it ealrier makes the conditions simpler (we don't have to check for
(vectorize && vectorarg) later).

The whole code block has been moved as-is with no changes apart from
whitespace.