pad.c:pad_reset: Also make sure the thing is not PADMY

pad.c:pad_reset: Also make sure the thing is not PADMY

We can’t SvPADTMP_off on just anything, because it asserts that the
thing is !SvPADMY.

A range operator has a target of sorts, but that target must not be
shared with any other operators; hence it gets allocated like a
‘my’ variable.  The target thus ends up with the SvPADMY flag set.  We
obviously don’t want to mark those targets as reusable in pad_reset,
so just skip them.

This gets things mostly working under USE_BROKEN_PAD_RESET without
threads, but a few tests still fail.