Fix #12095: bug_monitor_copy() should check users exist

bug_monitor_copy() assumes that all user IDs from the source bug are
still valid. We should first check to make sure that users from the
source bug still exist before adding them to the monitor list of the
destination bug.

core/bug_api.php

@@ -1769,7 +1769,8 @@ function bug_monitor_copy( $p_source_bug_id, $p_dest_bug_id ) {
 
 	for( $i = 0; $i < $t_count; $i++ ) {
 		$t_bug_monitor = db_fetch_array( $result );
-		if ( !user_is_monitoring_bug( $t_bug_monitor['user_id'], $c_dest_bug_id ) ) {
+		if ( user_exists( $t_bug_monitor['user_id'] ) &&
+			!user_is_monitoring_bug( $t_bug_monitor['user_id'], $c_dest_bug_id ) ) {
 			$query = 'INSERT INTO ' . $t_bug_monitor_table . ' ( user_id, bug_id )
 				VALUES ( ' . db_param() . ', ' . db_param() . ' )';
 			db_query_bound( $query, Array( $t_bug_monitor['user_id'], $c_dest_bug_id ) );