make perl -Dt display padnames with sort blocks

When a sort block (as opposed to sort sub) is executed, a new stackinfo is
pushed with a single CXt_NULL on top. Since S_deb_curcv() only examines
the *current* CX stack looking for the current running CV, it fails to
find it in this case and returns null.

This means that on threaded builds you get things like:

    $ perl -Dt -e'my $x; @a=sort { $x } 1,2'
    ...
    (-e:1)	padsv([1])

where it can't find a pad to look up the name of the lexical at targ 1.

This commit makes S_deb_curcv() continue to the previous CX stack when it
finds it's on a PERLSI_SORT stackinfo. The output from the above is now:

    (-e:1)	padsv($x)

0 files changed, 0 insertions, 0 deletions