[arabic/stch] Zero advances

This was working for our test suite because those glyphs were
declared mark and their advances zeroed automatically. But is
not the case in eg. the font posted at:

https://github.com/harfbuzz/harfbuzz/issues/4369

src/hb-ot-shaper-arabic.cc

@@ -599,6 +599,7 @@ apply_stch (const hb_ot_shape_plan_t *plan HB_UNUSED,
 
 	  DEBUG_MSG (ARABIC, nullptr, "appending %u copies of glyph %u; j=%u",
 		     repeat, info[k - 1].codepoint, j);
+	  pos[k - 1].x_advance = 0;
 	  for (unsigned int n = 0; n < repeat; n++)
 	  {
 	    x_offset -= width;