Ignore $subjects that cannot be multibyte split (#239)

[andrewryantech]

This appears to occur when the database field contains binary data. Some wordpress plugins do this, eg Wordfence

It appears there are no unit test to cover this case. I considered adding them, but looks like there's a bit of work to get them modernised. Would rather not sink time into that unless the maintainers are on board.

Considered using mb_detect_encoding, but it appears there are cases where mb_detect_encoding can detect a valid encoding for which mb_split still returns false.

Not sure if there are cases where we do really want to modify binary data, but seems like a pretty risky edge case imo.

[stakeworks]

public function get_columns( $table ) {
    $primary_key = array();
    $columns = array( );

    // Get a list of columns in this table
    $fields = $this->db_query( "DESCRIBE {$table}" );
    if ( ! $fields ) {
        $this->add_error( $this->db_error( ), 'db' );
    } else {
        while( $column = $this->db_fetch( $fields ) ) {
            if ( false !== stristr( $column['Type'], 'binary' )){
                $this->add_error( 'Skip field ' . $table . ' - ' . $column[ 'Field' ] . ', fieldtype is ' . $column['Type'] , 'results' );
                continue;
            }

            $columns[] = $column[ 'Field' ];
            if ( $column[ 'Key' ] == 'PRI' )
                $primary_key[] = $column[ 'Field' ];
        }
    }

    return array( $primary_key, $columns );
}

[harryfear]

Great, does this mean the problem's fixed? :)