skip creating new capture COW SV if possible

skip creating new capture COW SV if possible

Each time we do a match, we currently (where possible) make a COW copy of
the just-matched string. This involves creating a new SV that shares the
same PVX buffer with the string. In a repeated match like while (/.../g),
that means the each time round we free the old capture SV and create a new
one.

As as optimisation, skip the free/create if the old capture SV is already
a COW clone of the match string.