[perl #113576] Restore err msg for &{+undef}

This ‘Can't use an undefined value as a subroutine reference’ message
used to occur for &{+undef}, until commit 15ff848f77796 in 1997.

Then it started happening only for magic variables.

Non-magical variables would treat it the same as "", possibly dying
with ‘Undefined subroutine main:: called’, and possibly calling the
subroutine with that name.

Magical variables (&{$tied_undef}) started behaving the same way
(treating undef as "") with commit 7c75014e, in 2010.

This commit restores that message.

1 files changed, 2 insertions, 2 deletions

diff --git a/pp_hot.c b/pp_hot.c
index 38c49a068a..b2fc84ebf9 100644
--- a/pp_hot.c
+++ b/pp_hot.c
@@ -2601,9 +2601,9 @@ PP(pp_entersub)
 	else {
 	    const char *sym;
 	    STRLEN len;
-	    sym = SvPV_nomg_const(sv, len);
-	    if (!sym)
+	    if (!SvOK(sv))
 		DIE(aTHX_ PL_no_usym, "a subroutine");
+	    sym = SvPV_nomg_const(sv, len);
 	    if (PL_op->op_private & HINT_STRICT_REFS)
 		DIE(aTHX_ "Can't use string (\"%" SVf32 "\"%s) as a subroutine ref while \"strict refs\" in use", sv, len>32 ? "..." : "");
 	    cv = get_cvn_flags(sym, len, GV_ADD|SvUTF8(sv));