gv:gv_try_downgrade: Leave PL_stderrgv alone

gv_try_downgrade exists to remove globs and subs that were (possibly
temporarily) vivified by bareword lookup.  It is called whenever a
gvop is freed and its gv looks like a candidate for downgrading.  That
means it applies, not only to potential sub calls, but also to *foo
and *bar.  gv_try_downgrade may delete a glob from the stash alto-
gether if it is empty.  So eval "*foo if 0" may delete the *foo glob.

PL_stderrgv is a shortcut variable that points to *STDERR.  When
PL_stderrgv is null, the real stderr is used as a fallback.

If gv_try_downgrade deletes *STDERR (which causes PL_stderrgv to be
cleared), it can could the behaviour, which is not supposed to happen.
It doesn’t actually delete *STDERR right now, because undef() vivifies
the scalar slot, disqualifying the glob from downgrading.

If we change undef not to vivify the scalar slot (I plan to soon, and
this commit prepares for it), then this is what would happen without
this commit:

$ perl -le 'close *{"STDERR"}; undef *{"STDERR"}; eval "*STDERR if 0"; *{"STDERR"} = *STDOUT{IO}; warn'
$ perl -le 'close *{"STDERR"}; undef *{"STDERR"}; *{"STDERR"} = *STDOUT{IO}; warn'
Warning: something's wrong at -e line 1.

Notice how the first one-liner (with the eval) produces no output.

1 files changed, 1 insertions, 1 deletions

diff --git a/gv.c b/gv.c
index 70e638efba..93e6ccc95b 100644
--- a/gv.c
+++ b/gv.c
@@ -3320,7 +3320,7 @@ Perl_gv_try_downgrade(pTHX_ GV *gv)
 	    !GvSV(gv) && !GvAV(gv) && !GvHV(gv) && !GvIOp(gv) && !GvFORM(gv) &&
 	    GvEGVx(gv) == gv && (stash = GvSTASH(gv))))
 	return;
-    if (gv == PL_statgv) return;
+    if (gv == PL_statgv || gv == PL_stderrgv) return;
     if (SvMAGICAL(gv)) {
         MAGIC *mg;
 	/* only backref magic is allowed */