the wrong buffer

handler::index_next_same() did not take into account that the
internally called function key_cmp_if_same() uses the fixed
buffer table->record[0] for key comparison instead of the
buffer provided by the caller of handler::index_next_same().

Added code to temporarily redirect table->record[0] and the fields
used for the key to the record buffer provided by the caller of
handler::index_next_same().

The test case is in partition.test already.