Don’t clobber all magic when clobbering vstring

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?