Define $constaint is all cases in get_attachment_icon() to fix E_NOTICE. Fixed #5159 props richcon.

git-svn-id: http://svn.automattic.com/wordpress/trunk@6201 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/post-template.php

@@ -445,6 +445,7 @@ function get_attachment_icon( $id = 0, $fullsize = false, $max_dims = false ) {
 			}
 		} else {
 			$post->iconsize = array($imagesize[0], $imagesize[1]);
+			$constraint = '';
 		}
 	}