FAB18529 added nil check in channel header parsing
FAB18529 added nil check in channel header parsing

Fuzz testing has reported SEGV while sending incomplete/null
message request to orderer.

Signed-off-by: Parameswaran Selvam <34419c96699e413317b495e16a44e58e2d92c899@in.ibm.com>

manish-sethi/fabric-sidedb,manish-sethi/fabric,manish-sethi/fabric-sidedb,jimthematrix/fabric,manish-sethi/fabric-sidedb,manish-sethi/fabric-sidedb,hyperledger/fabric,hyperledger/fabric,jimthematrix/fabric,manish-sethi/fabric-sidedb,manish-sethi/fabric,manish-sethi/fabric-sidedb,manish-sethi/fabric-sidedb