[khmer] Fix infinite-loop in cluster merging

Indic shaper already had this check.  We removed it when forking
Khmer shaper by mistake.

Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1464623

src/hb-ot-shape-complex-khmer.cc

@@ -372,6 +372,9 @@ initial_reordering_consonant_syllable (const hb_ot_shape_plan_t *plan,
 	break;
       }
 
+    if (unlikely (end - start >= 127))
+      buffer->merge_clusters (start, end);
+    else
       /* Note!  syllable() is a one-byte field. */
       for (unsigned int i = base; i < end; i++)
 	if (info[i].syllable() != 255)