[indic] Fix infinite loop

Fixes https://bugs.chromium.org/p/chromium/issues/detail?id=863044
diff --git a/src/hb-ot-shape-complex-indic.cc b/src/hb-ot-shape-complex-indic.cc
index f145a34..918e0d7 100644
--- a/src/hb-ot-shape-complex-indic.cc
+++ b/src/hb-ot-shape-complex-indic.cc
@@ -789,7 +789,7 @@
      * We could use buffer->sort() for this, if there was no special
      * reordering of pre-base stuff happening later...
      */
-    if (indic_plan->is_old_spec || end - base > 127)
+    if (indic_plan->is_old_spec || end - start > 127)
       buffer->merge_clusters (base, end);
     else
     {