pad.c:pad_reset: check the name

pad.c:pad_reset: check the name

This code was sufficiently out-of-date to trip assertions in miniperl,
breaking the build (with -Accflags=-DUSE_BROKEN_PAD_RESET).

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

So check that it has no name first.  A pad entry without a name is an
operator target (or an unallocated entry).

The build is still broken, but it gets a bit further.