fixed CRL check return value on revoked certificates

fixed possible refcounting bugs
generic return_null() implementation

src/libstrongswan/utils.h

@@ -209,6 +209,11 @@ void *clalloc(void *pointer, size_t size);
  */
 void memxor(u_int8_t dest[], u_int8_t src[], size_t n);
 
+/**
+ * returns null
+ */
+void *return_null();
+
 /**
  * Special type to count references
  */