This is addition to fix for bug21617. Valgrind reports an error when
opening merge table that has underlying tables with less indexes than
in a merge table itself.

Copy at most min(file->keys, table->key_parts) elements from rec_per_key array.
This fixes problems when merge table and subtables have different number of keys.


sql/ha_myisammrg.cc:
  Copy at most min(file->keys, table->key_parts) elements from rec_per_key array.
  This fixes problems when merge table and subtables have different number of keys.