B: honor the UTF-8-ness of the label in B::COP::label

Fixes #24040

MANIFEST

@@ -4755,6 +4755,7 @@ ext/B/Makefile.PL			Compiler backend makefile writer
 ext/B/O.pm				Compiler front-end module (-MO=...)
 ext/B/t/b.t				See if B works
 ext/B/t/B/success.pm			Test module for ext/B/t/o.t
+ext/B/t/b_uni.t				See if B works with Unicode
 ext/B/t/bool.t				See if B works for bool
 ext/B/t/concise.t			See whether B::Concise works
 ext/B/t/concise-xs.t			See whether B::Concise recognizes XS functions

ext/B/B.xs

@@ -1037,7 +1037,15 @@ next(o)
 		    ret = newSVpvn_flags(cPVOPo->op_pv, strlen(cPVOPo->op_pv), SVs_TEMP);
 		break;
 	    case 42: /* B::COP::label */
-		ret = sv_2mortal(newSVpv(CopLABEL(cCOPo),0));
+                {
+                    STRLEN len;
+                    U32 flags;
+                    const char *pv = CopLABEL_len_flags(cCOPo, &len, &flags);
+                    ret = sv_2mortal(newSVpvn(pv, len));
+                    SvUTF8_off(ret);
+                    if (flags & SVf_UTF8)
+                        SvUTF8_on(ret);
+                }
 		break;
 	    case 43: /* B::COP::arybase */
 		ret = sv_2mortal(newSVuv(0));

ext/B/t/b_uni.t

@@ -0,0 +1,39 @@
+#!./perl
+
+BEGIN {
+    unshift @INC, 't';
+    require Config;
+    if (($Config::Config{'extensions'} !~ /\bB\b/) ){
+        print "1..0 # Skip -- Perl configured without B module\n";
+        exit 0;
+    }
+}
+
+$|  = 1;
+use warnings;
+use strict;
+use utf8;
+use B;
+BEGIN  {
+    eval { require threads; threads->import; }
+}
+use Test::More;
+
+sub f {
+    # I like pi
+    π:1;
+}
+
+{
+    # github 24040
+    my $f = B::svref_2object(\&f);
+    my $op = $f->START;
+    while ($op && !($op->name =~ /^(db|next)state$/ && $op->label)) {
+        $op = $op->next;
+    }
+    $op or die "Not found";
+    my $label = $op->label;
+    is($label, "π", "UTF8 label correctly UTF8");
+}
+
+done_testing();