Don’t crash with &CORE::foo after undefining *_

When a sub is called with & and no parentheses, no @_ is set up.  This
means the sub call sees the existing @_.  It also means that, if *_
has been undefined, there is no @_.

pp_coreargs was not accounting for this, and was doing
AvARRAY(GvAV(PL_defgv)) without checking that GvAV(PL_defgv) was non-
null.  It crashed as a result.

1 files changed, 2 insertions, 2 deletions

diff --git a/pp.c b/pp.c
index ba3ac1f0bf..ee82cd2c3a 100644
--- a/pp.c
+++ b/pp.c
@@ -5881,8 +5881,8 @@ PP(pp_coreargs)
     int opnum = SvIOK(cSVOP_sv) ? (int)SvUV(cSVOP_sv) : 0;
     int defgv = PL_opargs[opnum] & OA_DEFGV, whicharg = 0;
     AV * const at_ = GvAV(PL_defgv);
-    SV **svp = AvARRAY(at_);
-    I32 minargs = 0, maxargs = 0, numargs = AvFILLp(at_)+1;
+    SV **svp = at_ ? AvARRAY(at_) : NULL;
+    I32 minargs = 0, maxargs = 0, numargs = at_ ? AvFILLp(at_)+1 : 0;
     I32 oa = opnum ? PL_opargs[opnum] >> OASHIFT : 0;
     bool seen_question = 0;
     const char *err = NULL;