Fix for redefined sort subs nastiness

Fix for redefined sort subs nastiness

>sub sortfunc { &once }
>sub once {
>    eval q{
>	print "Eval from ", join(':', caller(0)), "\n";
>	sub rest {
>	    print "REST: $a <=> $b\n";
>	    $a <=> $b
>	}
>    };
>    *sortfunc = *rest;
>    &sortfunc;
>}
>@x = sort sortfunc 10, 5, 2.5, 1.25;

That misbehaves due to the redefinition of the sort sub
while the sort is active.  That's a big no-no, because
the sortcop was pointing to the CvSTART of the original,
and it will now point to freed memory (if the sub really
got undefined).

Here's a tested patch that does nothing but die under
that circumstance.

p5p-msgid: 199705090004.UAA15032@aatma.engin.umich.edu