Perl_multideref_stringify: don't SEGV on null cv

This function is called by e.g. "perl -Dt" to display the multideref op:

    $ perl -Dt -e'$a->{foo}[1]'
    ...
    (-e:1)	multideref($a->{"foo"}[1])

On threaded builds, it needs to know the correct pad (and so the correct
cv too) so that it can access GVs and const SVs that have been moved to
the pad.

However with a sort code block (rather than a sort sub), S_deb_curcv()
returns null, so multideref_stringify() is called with a null CV. This
then SEGVs.

Although ideally S_deb_curcv() should be fixed, a function like
multideref_stringify(), which can be used for debugging, should be robust
in unexpected circumstances. So this commit makes it safe (although not
particularly useful) with a null CV:

    $ perl -Dt -e'@a = sort { $a->[$i] <=> $b->[$i] } [0], [1]'
    ...
    (-e:1)	sort
    (-e:1)	multideref(<NULLGV>->[<NULLGV>])
    (-e:1)	multideref(<NULLGV>->[<NULLGV>])

1 files changed, 2 insertions, 3 deletions

diff --git a/proto.h b/proto.h
index af9648ef23..aa43b9504f 100644
--- a/proto.h
+++ b/proto.h
@@ -2770,10 +2770,9 @@ PERL_CALLCONV SV*	Perl_mro_set_private_data(pTHX_ struct mro_meta *const smeta,
 	assert(smeta); assert(which); assert(data)
 
 PERL_CALLCONV SV*	Perl_multideref_stringify(pTHX_ const OP* o, CV *cv)
-			__attribute__nonnull__(pTHX_1)
-			__attribute__nonnull__(pTHX_2);
+			__attribute__nonnull__(pTHX_1);
 #define PERL_ARGS_ASSERT_MULTIDEREF_STRINGIFY	\
-	assert(o); assert(cv)
+	assert(o)
 
 PERL_CALLCONV NV	Perl_my_atof(pTHX_ const char *s)
 			__attribute__nonnull__(pTHX_1);