Fix panic/crash with sort { $not_num } and fatal warnings

Fix panic/crash with sort { $not_num } and fatal warnings

I caused this in 5.15.4 in commit 1aa032b25ab:

$ ./miniperl -Ilib -e 'eval q|use warnings FATAL=>all=>; ()=sort{undef}1,2|'
panic: illegal pad in SAVEt_FREEOP: 0x803500[0x0] at -e line 1.

This panic only happens under debugging builds.

But it’s worse than that:

$ ./miniperl -Ilib -e 'eval { use warnings FATAL => all=>; ()=sort{undef}1,2}; my $x'
Bus error

It’s this piece of code in pp_sort.c that is the problem:

    pad = PL_curpad; PL_curpad = 0;
    if (PL_stack_sp != PL_stack_base + 1) {
	assert(PL_stack_sp == PL_stack_base);
	result = SvIV(&PL_sv_undef);
    }
    else result = SvIV(*PL_stack_sp);
    PL_curpad = pad;

If SvIV dies, then PL_curpad will never be restored.  That results in
a panic error when the string eval exits, under debugging builds, and
a crash for any subsequent pad ops, under any build.

So we need to use the savestack to protect PL_curpad.  To avoid the
overhead most of the time, we should do this only if the result is not
already a number.

Sorting with a sub that has a ($$) prototype follows a different
code path that contains the same logic, but it is safe in that case,
because sort with a sub already localises the pad.  I added tests for
it anyway.