DS-MRR improvements

- buildbot test failure fixes: don't try to get more records from SimpleIndexReader
  if we've already got EOF for it.

sql/multi_range_read.cc

@@ -303,6 +303,7 @@ int Mrr_simple_index_reader::init(handler *h_arg, RANGE_SEQ_IF *seq_funcs,
 {
   HANDLER_BUFFER no_buffer = {NULL, NULL, NULL};
   h= h_arg;
+  res= 0;
   return h->handler::multi_range_read_init(seq_funcs, seq_init_param, n_ranges,
                                            mode, &no_buffer);
 }
@@ -559,6 +560,9 @@ int Mrr_ordered_rndpos_reader::refill_buffer()
 
   last_identical_rowid= NULL;
 
+  if (index_reader->eof())
+    DBUG_RETURN(0);
+
   while (rowid_buffer->can_write())
   {
     res= index_reader->get_next(&range_info);

sql/multi_range_read.h

@@ -126,8 +126,8 @@ public:
   index tuple or a table record.
 
   Getting HA_ERR_END_OF_FILE from get_next() means that the source should be
-  re-filled. if eof() returns true after refill attempt, then end of stream has
-  been reached and get_next() must not be called anymore.
+  re-filled. if eof() returns true after refill attempt, then the end of 
+  stream has been reached and get_next() must not be called anymore.
 */
 
 class Mrr_reader