hlr.c: check the presence of msgb->l2h in read_cb()

Checking the presence of msgb->l2h in read_cb_forward() doesn't
make sense, since in read_cb() we pass it to osmo_gsup_decode().
Let's rather do this before calling osmo_gsup_decode().

Fix for Change-Id: Ia4f345abc877baaf0a8f73b8988e6514d9589bf5
Change-Id: I69a3d31aacbbb1abef3d83e42e46c899fe2f914b
1 file changed