libmsc/gsm_04_11.c: do not abuse LOG_TRANS() in gsm411_alloc_mt_trans()

This change is similar to I6b68a0f0b32eb126e0f7e914a314130254d28467.
If we 100% sure that trans == NULL, it makes more sense to use
generic LOGP(DLSMS, LOGL_*, ...) call, so the logs can reflect
more information than such dummy prefix:

  trans(NULL NULL callref-0x0 tid-0) ...

Change-Id: I3c1e633aee5dd7cd0d367404a3def9cffe0b3baa
diff --git a/src/libmsc/gsm_04_11.c b/src/libmsc/gsm_04_11.c
index 3a3ec8c..367cc6f 100644
--- a/src/libmsc/gsm_04_11.c
+++ b/src/libmsc/gsm_04_11.c
@@ -1097,13 +1097,13 @@
 					       struct vlr_subscr *vsub)
 {
 	struct msc_a *msc_a;
-	struct gsm_trans *trans = NULL;
+	struct gsm_trans *trans;
 	int tid;
 
 	/* Generate a new transaction ID */
 	tid = trans_assign_trans_id(net, vsub, TRANS_SMS);
 	if (tid == -1) {
-		LOG_TRANS(trans, LOGL_ERROR, "No available transaction IDs\n");
+		LOGP(DLSMS, LOGL_ERROR, "No available transaction IDs\n");
 		return NULL;
 	}