[GSUB] Fix reverse lookup loop like we did in the old code before

src/hb-ot-layout-gsub-private.h

@@ -808,7 +808,7 @@ struct SubstLookup : Lookup {
 	  else
 	    buffer->in_pos--;
 
-	} while (buffer->in_pos);
+	} while ((int) buffer->in_pos >= 0);
     }
 
     return ret;