fix utf8 length magic handling for scalar reverse

fixes #17737

Author: tonycoz

pp.c

@@ -5898,6 +5898,7 @@ PP(pp_reverse)
 	    sv_setsv(TARG, DEFSV);
 	    XPUSHs(TARG);
 	}
+        SvSETMAGIC(TARG); /* remove any utf8 length magic */
 
 	up = SvPV_force(TARG, len);
 	if (len > 1) {

t/op/reverse.t

@@ -6,7 +6,7 @@ BEGIN {
     set_up_inc('../lib');
 }
 
-plan tests => 24;
+plan tests => 25;
 
 is(reverse("abc"), "cba", 'simple reverse');
 
@@ -92,6 +92,15 @@ use Tie::Array;
     is($a, $c, 'Unicode string double reversal matches original');
 }
 
+{
+    # https://github.com/Perl/perl5/issues/17737
+    # utf8 length magic handling
+    # threw an assertion failure with -DDEBUGGING
+    my @x;
+    push @x, length reverse for split "-", "\x{100}--0";
+    is($x[1], 0, "check set magic being called to clear length magic");
+}
+
 # [perl #132544] stack pointer used to go wild when nullary reverse
 # required extending the stack
 for(0..1000){()=(0..$_,scalar reverse )}