Ignore un-initialized result on ReScan
If we run as the inner node of a nested loop mysqlReScanForeignScan
may be called first, prior to the result being initialized, causing a
segfault in the back end. There's nothing to be done in that case
anyway, so just ignore it.
mysql_row_seek() expects a MYSQL_ROW_OFFSET object rather than the
(first) row index number we were giving it. Use mysql_data_seek()