SimpleSAML_Utilities: Better error handling when opening /dev/urandom.

This code would create a loop in the error handler under windows.
Instead of trying to open the file, and handle the error, instead
check whether the file exists first.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@1715 44740490-163a-0410-bde0-09ae8108e29a

lib/SimpleSAML/Utilities.php

@@ -1122,7 +1122,11 @@ class SimpleSAML_Utilities {
 		assert('is_int($length)');
 
 		if($fp === NULL) {
-			$fp = @fopen('/dev/urandom', 'rb');
+			if (file_exists('/dev/urandom')) {
+				$fp = fopen('/dev/urandom', 'rb');
+			} else {
+				$fp = FALSE;
+			}
 		}
 
 		if($fp !== FALSE) {