chaintopology: always initialize smoothed feerate if it's the first entry.

Not just during startup: we could have bitcoind not give estimates until
later, but we don't want to smooth with zero.

The test changes in next patch trigger this, so I didn't write a test
with this patch.

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

lightningd/chaintopology.c

@@ -309,7 +309,7 @@ static void update_feerates(struct bitcoind *bitcoind,
 			continue;
 
 		/* Initial smoothed feerate is the polled feerate */
-		if (topo->feerate_uninitialized) {
+		if (!topo->feerate[i]) {
 			old_feerates[i] = feerate;
 			log_debug(topo->log,
 					  "Smoothed feerate estimate for %s initialized to polled estimate %u",