fix for [perl #65582] anon globs segfaulting

fix for [perl #65582] anon globs segfaulting

The following code has had differing behaviours:

    my $io_ref = *STDOUT{IO};
    my $glob = *$io_ref;

          defined($glob)    "$glob"
          --------------    -------
5.8.8       false           "" with uninit warning
5.10.0      true            (coredump)
this commit true            ""

$glob is essentially an anonymous typeglob (no NAME, EGV or GvSTASH).
It shouldn't register as undefined since it's clearly a valid GV with a
valid IO slot; Stringifying to "" seems to be the right thing, and not
warning seems right too, since its not undef.