check if deliverTxResCh is still open, return an err otherwise

Fixes #2408

CHANGELOG_PENDING.md

@@ -46,3 +46,4 @@ program](https://hackerone.com/tendermint).
     - keep accums averaged near 0
 - [types] \#2941 Preserve val.Accum during ValidatorSet.Update to avoid it being
   reset to 0 every time a validator is updated
+- [rpc] \#2408 `/broadcast_tx_commit`: Fix interface conversion: interface {} in nil, not EventDataTx error

rpc/core/mempool.go

@@ -198,7 +198,10 @@ func BroadcastTxCommit(tx types.Tx) (*ctypes.ResultBroadcastTxCommit, error) {
 	// TODO: configurable?
 	var deliverTxTimeout = rpcserver.WriteTimeout / 2
 	select {
-	case deliverTxResMsg := <-deliverTxResCh: // The tx was included in a block.
+	case deliverTxResMsg, ok := <-deliverTxResCh: // The tx was included in a block.
+		if !ok {
+			return nil, errors.New("Error on broadcastTxCommit: expected DeliverTxResult, got nil. Did the Tendermint stop?")
+		}
 		deliverTxRes := deliverTxResMsg.(types.EventDataTx)
 		return &ctypes.ResultBroadcastTxCommit{
 			CheckTx:   *checkTxRes,