Revert: 064309df1a "peer: signature in commit_info is always valid."

It's not quite true: if we offer the anchor, we have a commitinfo
without their signature yet.  So make it a pointer again.  Since we
always allocate struct commit_info with talz, it starts as a NULL
pointer.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

daemon/peer.c

@@ -1294,7 +1294,7 @@ const struct bitcoin_tx *bitcoin_commit(struct peer *peer)
 
 	peer->us.commit->tx->input[0].witness
 		= bitcoin_witness_2of2(peer->us.commit->tx->input,
-				       &peer->us.commit->sig,
+				       peer->us.commit->sig,
 				       &sig,
 				       &peer->them.commitkey,
 				       &peer->us.commitkey);