OSMO_SOCKADDR_STR_FMT_ARGS: guard against NULL pointer

The pointless '(R)->ip?' condition of the previous commit made me want to
protect against R == NULL instead.

Change-Id: Ie2f47ad8ae585aaf67a6476c67f8e014820a72bc
diff --git a/include/osmocom/core/sockaddr_str.h b/include/osmocom/core/sockaddr_str.h
index 7c01732..d16dc86 100644
--- a/include/osmocom/core/sockaddr_str.h
+++ b/include/osmocom/core/sockaddr_str.h
@@ -62,7 +62,7 @@
  *     printf("got " OSMO_SOCKADDR_STR_FMT "\n", OSMO_SOCKADDR_STR_FMT_ARGS(my_sockaddr_str));
  */
 #define OSMO_SOCKADDR_STR_FMT "%s:%u"
-#define OSMO_SOCKADDR_STR_FMT_ARGS(R) (R)->ip, (R)->port
+#define OSMO_SOCKADDR_STR_FMT_ARGS(R) ((R)? (R)->ip : "NULL"), ((R)? (R)->port : 0)
 
 bool osmo_sockaddr_str_is_set(const struct osmo_sockaddr_str *sockaddr_str);