C: fix $/ by checking GMG (get magic)

some PVs returned undef instead of their magic value, like PL_rs ($/).
We should get this value by $sv->PV but this only calls SvPVX_const, not the proper SvPV_const macro.
So we need to let perl expand the value for us.
This should not happen for IV and NVs, I hope (TODO check)