Perl_sv_vcatpvfn_flags: simplify wrap checking

Perl_sv_vcatpvfn_flags: simplify wrap checking

The main SvGROW() has a new-length arg roughly equivalent to

    (SvCUR(sv) + elen + zeros + esignlen + dotstrlen + 1);

Rationalise the overflow/wrap checking by doing each individual addition
separately with its own check. This is slightly redundant as some of the
values are interdependent, but this way it's easier to see whether all
possible overflows are being checked for.

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