Revert "[GSUB/GPOS] Reject Context/ChainContext lookups with zero input"

This reverts commit 0981068b.

I was confused.  Even if we access coverage[0] unconditionally, we don't
need bound checks since the array machinary already handles that.

src/hb-ot-layout-gsubgpos-private.hh

@@ -928,7 +928,6 @@ struct ContextFormat3
     TRACE_SANITIZE ();
     if (!c->check_struct (this)) return TRACE_RETURN (false);
     unsigned int count = glyphCount;
-    if (unlikely (!glyphCount)) return TRACE_RETURN (false);
     if (!c->check_array (coverage, coverage[0].static_size, count)) return TRACE_RETURN (false);
     for (unsigned int i = 0; i < count; i++)
       if (!coverage[i].sanitize (c, this)) return TRACE_RETURN (false);
@@ -1467,7 +1466,6 @@ struct ChainContextFormat3
     if (!backtrack.sanitize (c, this)) return TRACE_RETURN (false);
     OffsetArrayOf<Coverage> &input = StructAfter<OffsetArrayOf<Coverage> > (backtrack);
     if (!input.sanitize (c, this)) return TRACE_RETURN (false);
-    if (unlikely (!input.len)) return TRACE_RETURN (false);
     OffsetArrayOf<Coverage> &lookahead = StructAfter<OffsetArrayOf<Coverage> > (input);
     if (!lookahead.sanitize (c, this)) return TRACE_RETURN (false);
     ArrayOf<LookupRecord> &lookup = StructAfter<ArrayOf<LookupRecord> > (lookahead);