ipa: Set msgb->l1h to the beginning of the IPA header

l2h points to the OML/RSL header, so it is only logical that l1h points
to the IPA header. Also, this is what osmo-bts/msg_utils.c expects.
diff --git a/src/input/ipa.c b/src/input/ipa.c
index 71e1227..056c648 100644
--- a/src/input/ipa.c
+++ b/src/input/ipa.c
@@ -64,8 +64,10 @@
 	int len, ret;
 	int needed;
 
-	if (msg == NULL)
+	if (msg == NULL) {
 		msg = ipa_msg_alloc(0);
+		msg->l1h = msg->tail;
+	}
 
 	if (msg == NULL) {
 		ret = -ENOMEM;