Make PerlIO::encoding handle cows

Make PerlIO::encoding handle cows

Commits 667763bdbf and e9a8753af fixed bugs involving buffer realloca-
tions during encode and decode.  But what was not taken into account
was that the COW flags could still be left on even when buffer real-
ocations were accounted for.  This could result in SvPV_set and
SvLEN_set(sv,0) being called on an SV with the COW flags still on,
so SvPVX would be treated as a key inside a shared_he, resulting in
assertion failures.