Fix #10818: MetaColumnNames ADOdb function may return bool value

The MetaColumnNames ADOdb function doesn't always return an array - it
could return a boolean value of false as well. Therefore before using
the result of a call to MetaColumnNames, we must first check that we've
got a valid array. ADOdb documentation of course doesn't mention this.

core/database_api.php

@@ -602,8 +602,8 @@ function db_index_exists( $p_table_name, $p_index_name ) {
  */
 function db_field_exists( $p_field_name, $p_table_name ) {
 	global $g_db;
-	$columns = $g_db->MetaColumnNames( $p_table_name );
-	return is_array( $columns ) ? in_array( $p_field_name, $columns ) : false;
+	$columns = db_field_names( $p_table_name );
+	return in_array( $p_field_name, $columns );
 }
 
 /**
@@ -613,7 +613,8 @@ function db_field_exists( $p_field_name, $p_table_name ) {
  */
 function db_field_names( $p_table_name ) {
 	global $g_db;
-	return $g_db->MetaColumnNames( $p_table_name );
+	$columns = $g_db->MetaColumnNames( $p_table_name );
+	return is_array( $columns ) ? $columns : array();
 }
 
 /**

manage_user_page.php

@@ -68,7 +68,7 @@
 $c_filter = '';
 
 # Clean up the form variables
-if ( !in_array( $f_sort, db_field_names( $t_user_table ) ) ) {
+if ( !db_field_exists( $f_sort, $t_user_table ) ) {
 	$c_sort = 'username';
 } else {
 	$c_sort = addslashes($f_sort);