[perl #121366] avoid using an invalid SvPVX() in Perl_sv_pvn_force_flags

[perl #121366] avoid using an invalid SvPVX() in Perl_sv_pvn_force_flags

This would cause valgrind to complain about:

  vec($Foo, 0, 1) = 1; # for example

when $Foo was undef, since SvPVX()[1] isn't initialized until the SV is
at least a SVt_PV.

[1] well, sv_u.svu_rv, but sv_u is a union, so the same memory is
initialized.  This isn't technically legal from a C point of view,
but pointer types are compatible enough with each other for it to not
be an issue.