Galleries: Avoid doubling up clearing br elements.

props drozdz.
fixes #25537.

Built from https://develop.svn.wordpress.org/trunk@27800


git-svn-id: http://core.svn.wordpress.org/trunk@27635 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/media.php

@@ -996,8 +996,12 @@ function gallery_shortcode( $attr ) {
 			$output .= '<br style="clear: both" />';
 	}
 
+	if ( $columns > 0 && $i % $columns !== 0 ) {
+		$output .= "
+			<br style='clear: both' />";
+	}
+
 	$output .= "
-			<br style='clear: both;' />
 		</div>\n";
 
 	return $output;