lightningd: fix assert() if spawning openingd fails.

My node crashed as follows:

	lightningd: lightningd/peer_control.c:957: peer_connected: Assertion `!peer->uncommitted_channel' failed.

In the logs I found:

	Running lightning_openingd: Cannot allocate memory

Which reveals that we're not freeing uc in that path!

Changelog-Fixed: Fix assertion on reconnect if we fail to run openingd.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/opening_control.c

@@ -977,6 +977,7 @@ void peer_start_openingd(struct peer *peer,
 					       tal_fmt(tmpctx,
 						       "Running lightning_openingd: %s",
 						       strerror(errno)));
+		tal_free(uc);
 		return;
 	}