Fix events.test failure on manu pushbuild hosts (5.1-runtime).

sql/event_db_repository.cc:
  Fix events.test failure on many pushbuild hosts (5.1-runtime).
  Make sure that when reading mysql.event the key number matches
  the field number that we're using to read by key.
  Also, print an error in case of a handler error (to not return 
  'Unknown error' when a table is corrupted to the user).

sql/event_db_repository.cc

@@ -310,17 +310,27 @@ Event_db_repository::index_read_for_db_for_i_s(THD *thd, TABLE *schema_table,
 
   DBUG_PRINT("info", ("Using prefix scanning on PK"));
   event_table->file->ha_index_init(0, 1);
-  event_table->field[ET_FIELD_DB]->store(db, strlen(db), scs);
   key_info= event_table->key_info;
+
+  if (key_info->key_parts == 0 ||
+      key_info->key_part[0].field != event_table->field[ET_FIELD_DB])
+  {
+    /* Corrupted table: no index or index on a wrong column */
+    my_error(ER_CANNOT_LOAD_FROM_TABLE, MYF(0), "event");
+    ret= 1;
+    goto end;
+  }
+
+  event_table->field[ET_FIELD_DB]->store(db, strlen(db), scs);
   key_len= key_info->key_part[0].store_length;
 
   if (!(key_buf= (byte *)alloc_root(thd->mem_root, key_len)))
   {
-    ret= 1;
     /* Don't send error, it would be done by sql_alloc_error_handler() */
+    ret= 1;
+    goto end;
   }
-  else
-  {
+
   key_copy(key_buf, event_table->record[0], key_info, key_len);
   if (!(ret= event_table->file->index_read(event_table->record[0], key_buf,
                                            (key_part_map)1, HA_READ_PREFIX)))
@@ -335,13 +345,17 @@ Event_db_repository::index_read_for_db_for_i_s(THD *thd, TABLE *schema_table,
     } while (ret == 0);
   }
   DBUG_PRINT("info", ("Scan finished. ret=%d", ret));
-  }
-  event_table->file->ha_index_end();
+
   /*  ret is guaranteed to be != 0 */
   if (ret == HA_ERR_END_OF_FILE || ret == HA_ERR_KEY_NOT_FOUND)
-    DBUG_RETURN(FALSE);
+    ret= 0;
+  else
+    event_table->file->print_error(ret, MYF(0));
 
-  DBUG_RETURN(TRUE);
+end:
+  event_table->file->ha_index_end();
+
+  DBUG_RETURN(test(ret));
 }