smpp: Fix potential crash in handling submitSM

In case:

* No message_payload and a 0 sm_length was used
* esm_class indicates UDH being present
* 7bit encoding was requested

The code would execute:

  ud_len = *sms_msg + 1;

Which is a NULL pointer dereference and would lead
to a crash of the NITB. Enforce the limits of the
sm_length parameter and reject the messae otherwise.

Fixes: Coverity CID 1042373
diff --git a/openbsc/src/libmsc/smpp_openbsc.c b/openbsc/src/libmsc/smpp_openbsc.c
index ff5ab40..b17222f 100644
--- a/openbsc/src/libmsc/smpp_openbsc.c
+++ b/openbsc/src/libmsc/smpp_openbsc.c
@@ -114,12 +114,13 @@
 		}
 		sms_msg = t->value.octet;
 		sms_msg_len = t->length;
-	} else if (submit->sm_length) {
+	} else if (submit->sm_length > 0 && submit->sm_length < 255) {
 		sms_msg = submit->short_message;
 		sms_msg_len = submit->sm_length;
 	} else {
-		sms_msg = NULL;
-		sms_msg_len = 0;
+		LOGP(DLSMS, LOGL_ERROR,
+			"SMPP neither message payload nor valid sm_length.\n");
+		return ESME_RINVPARLEN;
 	}
 
 	sms = sms_alloc();