mess_sv(): access only if PL_curcop is non-null

RT #130621

In Perl_mess_sv(), don't try to add an "at foo line NN" to the error
message if PL_curcop is null.

In the ticket above, the reason that PL_curcop is null is the less
than optimal way that evals free their optree: ideally the optree should
be attached to the eval CV and freed when the CV is; instead a separate
SAVEFREEOP() is done. But that fix is for another time; regardless,
mess_sv() should have a PL_curcop != NULL guard anyway.

Author: iabyn

util.c

@@ -1518,14 +1518,17 @@ Perl_mess_sv(pTHX_ SV *basemsg, bool consume)
 	 * from the sibling of PL_curcop.
 	 */
 
-	const COP *cop =
-	    closest_cop(PL_curcop, OpSIBLING(PL_curcop), PL_op, FALSE);
-	if (!cop)
-	    cop = PL_curcop;
-
-	if (CopLINE(cop))
-	    Perl_sv_catpvf(aTHX_ sv, " at %s line %" IVdf,
-	    OutCopFILE(cop), (IV)CopLINE(cop));
+        if (PL_curcop) {
+            const COP *cop =
+                closest_cop(PL_curcop, OpSIBLING(PL_curcop), PL_op, FALSE);
+            if (!cop)
+                cop = PL_curcop;
+
+            if (CopLINE(cop))
+                Perl_sv_catpvf(aTHX_ sv, " at %s line %" IVdf,
+                                OutCopFILE(cop), (IV)CopLINE(cop));
+        }
+
 	/* Seems that GvIO() can be untrustworthy during global destruction. */
 	if (GvIO(PL_last_in_gv) && (SvTYPE(GvIOp(PL_last_in_gv)) == SVt_PVIO)
 		&& IoLINES(GvIOp(PL_last_in_gv)))