[FAB-7725]: add nil checks for events processing

The root cause behind of FAB-7725 is the fact that while dereferencing
certain fields after their desirialized, there is no proper check for
nil values, which might cause peer to crash once malstructured message
appears. This commit adds additional check as well as UT to cover the
issue described in FAB-7725.

Change-Id: I6b35aa6e62992a3e74e4b992d251675dc326f00f
Signed-off-by: Artem Barger <bartem@il.ibm.com>

core/peer/deliverevents.go

@@ -176,6 +176,11 @@ func (block *blockEvent) toFilteredBlock() (*peer.FilteredBlock, error) {
 			return nil, errors.WithMessage(err, "could not extract payload from envelope")
 		}
 
+		if payload.Header == nil {
+			logger.Debugf("transaction payload header is nil, %d, block num %d",
+				txIndex, block.Header.Number)
+			continue
+		}
 		chdr, err := utils.UnmarshalChannelHeader(payload.Header.ChannelHeader)
 		if err != nil {
 			return nil, err
@@ -215,6 +220,10 @@ func (ta transactionActions) toFilteredActions() (*peer.FilteredTransaction_Tran
 			return nil, errors.WithMessage(err, "error unmarshal transaction action payload for block event")
 		}
 
+		if chaincodeActionPayload.Action == nil {
+			logger.Debugf("chaincode action, the payload action is nil, skipping")
+			continue
+		}
 		propRespPayload, err := utils.GetProposalResponsePayload(chaincodeActionPayload.Action.ProposalResponsePayload)
 		if err != nil {
 			return nil, errors.WithMessage(err, "error unmarshal proposal response payload for block event")