fix COW match capture optimisation

fix COW match capture optimisation

When an SV matches, a new SV is created which is a COW copy of the original
SV, and stored in prog->saved_copy, then prog->subbeg is set to point to
the (shared) PVX buffer.

Earlier in this branch I introduced an optimisation that skipped freeing
the old saved_copy and creating a new COW SV each time, if the old
saved_copy SV was already a shared copy of the SV being matched.

So far so good, except that I implemented it wrongly: if non-COW
matches (which malloc() subbeg) are interspersed with COW matches,
then the subbeg of the COW and the malloced subbeg get mixed up and
AddressSanitizer throws a wobbly.

The fix is simple: in the optimised branch, we still need to free subbeg
if RXp_MATCH_COPIED is true, then reassign it.