Bug#16208709 - CRASH IN GET_SEL_ARG_FOR_KEYPART ON SELECT DISTINCT

	       ON COL WITH COMPOSITE INDEX

This problem is caused by the patch for the bug#11751794.
While checking for the keypart covering non grouping attribute. we are not 
checking whether the root node of the SEL_ARG* tree for the index have any 
cvalue or not.

sql/opt_range.cc

@@ -9870,6 +9870,8 @@ get_sel_arg_for_keypart(Field *nga_field,
                         SEL_ARG *keypart_tree,
                         SEL_ARG **cur_range)
 {
+  if(keypart_tree == NULL)
+    return false;
   if(keypart_tree->field->eq(nga_field))
   {
     /*