[morx] Leave ligature on stack after ligating

Test with Apple Color Emoji.ttf of ~66MB and following sequence:

./hb-shape Apple\ Color\ Emoji-old.ttf --font-funcs=ot -u U+1F468,200d,U+1F469,200d,U+1F467,200d,1f466

Should form full family if matching works correctly.  It first makes
family of three, then makes family of four out of that and the last
two characters.

Fixes https://github.com/harfbuzz/harfbuzz/issues/1292

src/hb-aat-layout-morx-table.hh

@@ -442,7 +442,6 @@ struct LigatureSubtable
 	  action_idx++;
 	}
 	while (!(action & LigActionLast));
-	match_length = 0;
 	buffer->move_to (end);
       }