add stack extend to Perl_die_unwind()

add stack extend to Perl_die_unwind()

When invoked via a scalar-context eval, die_unwind() was pushing
&PL_sv_undef without extending the stack first. It's been this way for
many years. Spotted by visual inspection while adding rpp_() code.

I could get a SEGV with the following code, but I haven't added it as a
test as it's highly sensitive to exactly what size stack is initially
allocated and the OS's malloc().

    my $x;
    my @a =
        ($x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,
         $x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,
         $x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,
         $x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,
         $x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,
         $x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,
         $x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,
         $x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,$x,
         scalar eval { die },
    );