[perl #106726] Don’t crash on length(@arr) warning

The RT ticket blames this on 676a678, but it was actually commit
579333e.  676a678 extended this problem to evals (and modules),
but it already occurred in the main program.

This crashes:

    ./miniperl -Ilib -we 'sub {length my @forecasts}'

because it is trying to find the variable name for the warning in the
CV returned by find_runcv, but this is a *compile-time* warning, so
using find_runcv is just wrong.

It ends up looking for the array in PL_main_cv’s pad, instead of
PL_compcv.

op.c

@@ -9720,7 +9720,8 @@ Perl_ck_length(pTHX_ OP *o)
                 case OP_PADHV:
                 case OP_PADAV:
                     name = varname(
-                        NULL, hash ? '%' : '@', kid->op_targ, NULL, 0, 1
+                        (GV *)PL_compcv, hash ? '%' : '@', kid->op_targ,
+                        NULL, 0, 1
                     );
                     break;
                 case OP_RV2HV:

sv.c

@@ -13859,7 +13859,7 @@ Perl_varname(pTHX_ const GV *const gv, const char gvtype, PADOFFSET targ,
 {
 
     SV * const name = sv_newmortal();
-    if (gv) {
+    if (gv && isGV(gv)) {
 	char buffer[2];
 	buffer[0] = gvtype;
 	buffer[1] = 0;
@@ -13878,10 +13878,12 @@ Perl_varname(pTHX_ const GV *const gv, const char gvtype, PADOFFSET targ,
 	}
     }
     else {
-	CV * const cv = find_runcv(NULL);
+	CV * const cv = gv ? (CV *)gv : find_runcv(NULL);
 	SV *sv;
 	AV *av;
 
+	assert(!cv || SvTYPE(cv) == SVt_PVCV);
+
 	if (!cv || !CvPADLIST(cv))
 	    return NULL;
 	av = MUTABLE_AV((*av_fetch(CvPADLIST(cv), 0, FALSE)));

t/op/length.t

@@ -6,7 +6,7 @@ BEGIN {
     @INC = '../lib';
 }
 
-plan (tests => 37);
+plan (tests => 38);
 
 print "not " unless length("")    == 0;
 print "ok 1\n";
@@ -224,4 +224,11 @@ is($ul, undef, "Assigned length of overloaded undef with result in TARG");
     print length undef;
 }
 
+{
+    local $SIG{__WARN__} = sub {
+	pass '[perl #106726] no crash with length @lexical warning'
+    };
+    eval ' sub { length my @forecasts } ';
+}
+
 is($warnings, 0, "There were no other warnings");