Don’t skip tied EXISTS for negative array indices

Don’t skip tied EXISTS for negative array indices

This was broken in 5.14.0 for those cases where $NEGATIVE_INDICES is
not true:

sub TIEARRAY{bless[]};
sub FETCHSIZE { 50 }
sub EXISTS { print "does $_[1] exist?\n" }
tie @a, "";
exists $a[1];
exists $a[-1];
$NEGATIVE_INDICES=1;
exists $a[-1];

$ pbpaste|perl5.12.0
does 1 exist?
does 49 exist?
does -1 exist?
$ pbpaste|perl5.14.0
does 1 exist?
does -1 exist?

This was broken by 54a4274e3c.