clear_defarray(): clear @_ if possible

clear_defarray(): clear @_ if possible

clear_defarray() is called during sub exit to clean up @_ in the
less common case where @_ has somehow gotten reified.

At the moment it just frees the old @_, then creates a new AV and
sticks it in pad[0].

This commit changes it so that for the reasonably common case of a reified
@_ where it's not magical and only has a reference count of 1, just call
av_clear() on it, rather than freeing and recreating.

Typical code that will benefit from this change is be something like:

    sub f { push @_, ...; ... }

which causes the AV to be reified, but not to become otherwise visible.