fix clique_sealer: reset block header coinbase

if we are not voting, coinbase should be filled with zero
because other subsystem e.g txpool can produce block header
with non zero coinbase. if that coinbase is one of the signer
and the nonce is zero, that signer will be vote out from
signer list

nimbus/p2p/clique/clique_sealer.nim

@@ -180,6 +180,13 @@ proc prepare*(c: Clique; parent: BlockHeader, header: var BlockHeader): CliqueOk
   if rc.isErr:
     return err(rc.error)
 
+  # if we are not voting, coinbase should be filled with zero
+  # because other subsystem e.g txpool can produce block header
+  # with non zero coinbase. if that coinbase is one of the signer
+  # and the nonce is zero, that signer will be vote out from
+  # signer list
+  header.coinbase.reset
+
   if (header.blockNumber mod c.cfg.epoch) != 0:
     c.doExclusively:
       # Gather all the proposals that make sense voting on