core/osmo_io: Fix reception of partial packets

Always append to 'msg->tail' instead of to 'msg->data'.

Change-Id: I0ab8028c807b4181fddd3c00ea2e037c40cf0813
diff --git a/src/core/osmo_io_poll.c b/src/core/osmo_io_poll.c
index dd86f29..c4819fa 100644
--- a/src/core/osmo_io_poll.c
+++ b/src/core/osmo_io_poll.c
@@ -56,7 +56,7 @@
 		}
 
 		hdr.msg = msg;
-		hdr.iov[0].iov_base = msgb_data(msg);
+		hdr.iov[0].iov_base = msg->tail;
 		hdr.iov[0].iov_len = msgb_tailroom(msg);
 		hdr.hdr.msg_iov = &hdr.iov[0];
 		hdr.hdr.msg_iovlen = 1;