Fix mssql summary error

When computing SUM & AVG functions, sqlserver uses the column
datatype, which is INT. This datatype can be overflowed when a big
number of issues are included, since we are adding the total number
of seconds.

In that case, use an explicit cast to BIGINT to avoid that error.
Other databases are not affected because they use a runtime data
type allowing bigger integers.

The stats array is initialized to zero, to return a properly formatted 
value array when there aren't any issues.

Fixes #25781
PR #1516

core/summary_api.php

@@ -1154,7 +1154,12 @@ function summary_helper_get_time_stats( $p_project_id, array $p_filter = null )
 	# (e.g. bug is CLOSED, not RESOLVED). The linkage to the history field
 	# will look up the most recent 'resolved' status change and return it as well
 
-	$t_stats = array();
+	$t_stats = array(
+		'bug_id'       => 0,
+		'largest_diff' => 0,
+		'total_time'   => 0,
+		'average_time' => 0,
+		);
 
 	$t_sql_inner = ' FROM {bug} b LEFT JOIN {bug_history} h'
 		. ' ON b.id = h.bug_id  AND h.type = :hist_type'
@@ -1175,7 +1180,14 @@ function summary_helper_get_time_stats( $p_project_id, array $p_filter = null )
 	}
 
 	if( db_has_capability( DB_CAPABILITY_WINDOW_FUNCTIONS ) ) {
-		$t_sql = 'SELECT id, diff, SUM(diff) OVER () AS total_time, AVG(diff) OVER () AS avg_time'
+		if(db_is_mssql() ) {
+			# sqlserver by default uses the column datatype, which is INT. This datatype can be overflowed
+			# when a big number of issues are included, since we are adding the total number of seconds.
+			$t_diff_expr = 'CAST(diff AS BIGINT)';
+		} else {
+			$t_diff_expr = 'diff';
+		}
+		$t_sql = 'SELECT id, diff, SUM(' . $t_diff_expr . ') OVER () AS total_time, AVG(' . $t_diff_expr . ') OVER () AS avg_time'
 			. ' FROM ( SELECT b.id, MAX(h.date_modified) - b.date_submitted AS diff'
 			. $t_sql_inner
 			. ' GROUP BY b.id,b.date_submitted ) subquery'