common: generalize extract_channel_id().

connectd is going to end up using this do demux; make it fast and complete.

Fixing this reveals a problem in openingd: it now extracts the channel_id
from funding_signed (which is where we transition off the temporary), and
gets upset.  So fix that.

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

common/read_peer_msg.c

@@ -103,6 +103,9 @@ bool is_peer_error(const tal_t *ctx, const u8 *msg,
 bool is_wrong_channel(const u8 *msg, const struct channel_id *expected,
 		      struct channel_id *actual)
 {
+	if (!expected)
+		return false;
+
 	if (!extract_channel_id(msg, actual))
 		return false;