NS2: follow the change of ownership

When receiving a primitive from the NS2 layer
the PCU must free the msg buffer if given.

Change-Id: I180433735bfbb3375c41318d7a7709d5845199ba
diff --git a/src/gprs_bssgp_pcu.cpp b/src/gprs_bssgp_pcu.cpp
index 413408d..671629b 100644
--- a/src/gprs_bssgp_pcu.cpp
+++ b/src/gprs_bssgp_pcu.cpp
@@ -597,7 +597,7 @@
 		LOGP(DPCU, LOGL_NOTICE, "NS: %s Unknown prim %d from NS\n",
 		     get_value_string(osmo_prim_op_names, oph->operation),
 		     oph->operation);
-		return 0;
+		goto out;
 	}
 
 	switch (oph->primitive) {
@@ -620,6 +620,10 @@
 		break;
 	}
 
+out:
+	if (oph->msg)
+		msgb_free(oph->msg);
+
 	return rc;
 }