Don’t clobber all magic when clobbering vstring

This code in sv_setsv, introduced in ece467f9b3, can’t possi-
bly be right:

    if ( SvVOK(dstr) )
    {
	/* need to nuke the magic */
	mg_free(dstr);
    }

And here is a test to prove it:

sub TIESCALAR { bless[]}
sub STORE {}
tie $@, "";
$@ = v0;
warn tied $@; # main=ARRAY(0xc0ffee)
$@ = 3;
warn tied $@; # something’s wrong

It blows away tiedness.

You could do that to any variable.

Let’s see:

$! = v0;
$! = 3;
open foo, 'oentuhaeontu' or die $!;  # 3 at - line 3.

Youch!

Let’s just free vstring magic, shall we?

1 files changed, 1 insertions, 1 deletions

diff --git a/sv.c b/sv.c
index 019e8dd11c..39ddbff86e 100644
--- a/sv.c
+++ b/sv.c
@@ -3949,7 +3949,7 @@ Perl_sv_setsv_flags(pTHX_ SV *dstr, register SV* sstr, const I32 flags)
     if ( SvVOK(dstr) )
     {
 	/* need to nuke the magic */
-	mg_free(dstr);
+	sv_unmagic(dstr, PERL_MAGIC_vstring);
     }
 
     /* There's a lot of redundancy below but we're going for speed here */