C: revert parts of the ptr_undef changes, fixes 91,263 missing stashnames

for PVMG entries (mainly magic ISA elements) the pv was not stored.
also optimize such set-magic entries to be static, as they do not grow, the set-magic assigns it.
use 0 instead of ptr_undef for normal PVs, just comppadnames need PL_sv_undef apparently