fix a memory leak in sv_sethek(), amending 70b71ec84

the following code reproduced this issue on perl 5.16.0:

my $o = bless {};
while (1) {
    for my $r([], $o) {
        ref $r;
    }
}

1 files changed, 1 insertions, 0 deletions

diff --git a/sv.c b/sv.c
index 1bccaae75a..b0eaf92304 100644
--- a/sv.c
+++ b/sv.c
@@ -4553,6 +4553,7 @@ Perl_sv_sethek(pTHX_ register SV *const sv, const HEK *const hek)
         {
 	    SV_CHECK_THINKFIRST_COW_DROP(sv);
 	    SvUPGRADE(sv, SVt_PV);
+	    Safefree(SvPVX(sv));
 	    SvPV_set(sv,(char *)HEK_KEY(share_hek_hek(hek)));
 	    SvCUR_set(sv, HEK_LEN(hek));
 	    SvLEN_set(sv, 0);