Upgrade: `$wpdb->get_col_length()` sanity check: bail on unexpected return value.

Merges [32429] for the 3.9 branch.

See #32165.

Built from https://develop.svn.wordpress.org/branches/3.9@32433


git-svn-id: http://core.svn.wordpress.org/branches/3.9@32403 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/includes/upgrade.php

@@ -1292,6 +1292,11 @@ function upgrade_397() {
 
 	if ( $wp_current_db_version < 27918 ) {
 		$content_length = $wpdb->get_col_length( $wpdb->comments, 'comment_content' );
+
+		if ( is_wp_error( $content_length ) ) {
+			return;
+		}
+
 		if ( false === $content_length ) {
 			$content_length = array(
 				'type'   => 'byte',