core: safety check for free contact

- patch by Sebastian Z., checking if the content is null before freeing
  it
(cherry picked from commit 7aed7d72c98a8228e9687773c3bab20d8934251a)

parser/contact/parse_contact.c

@@ -102,6 +102,8 @@ int parse_contact(struct hdr_field* _h)
  */
 void free_contact(contact_body_t** _c)
 {
+	if(*_c==NULL)
+		return;
 	if ((*_c)->contacts) {
 		free_contacts(&((*_c)->contacts));
 	}