bitcoin_tx_check: don't rely on tmpctx.

We assert() this in onchaind while grinding fees; better to free newtx.

Before this we hit 530MB, after a mere 2.5MB.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Fixed: onchaind uses much less memory on unilateral closes for old channels.

bitcoin/tx.c

@@ -256,6 +256,7 @@ bool bitcoin_tx_check(const struct bitcoin_tx *tx)
 	if (written != tal_bytelen(newtx))
 		return false;
 
+	tal_free(newtx);
 	return true;
 }