Fix NullReferenceException when outputting CustomObjectCommand error message

[argsno]

the error message is:

08::34::20 crit: Session[0] [198.18.0.1:3278] [127.0.0.1:58858] [020A9EF0] ProcessMessages threw exception System.NullReferenceException: Object reference not set to an instance of an object.    
at Garnet.server.RespServerSession.ProcessOtherCommands[TGarnetApi](Int32 count, TGarnetApi& storageApi) in /Users/argsno/Projects/garnet/libs/server/Resp/RespServerSession.cs:line 588    
at Garnet.server.RespServerSession.ProcessArrayCommands[TGarnetApi](TGarnetApi& storageApi) in /Users/argsno/Projects/garnet/libs/server/Resp/RespServerSession.cs:line 496    
at Garnet.server.RespServerSession.ProcessBasicCommands[TGarnetApi](Byte* ptr, RespCommand cmd, TGarnetApi& storageApi) in /Users/argsno/Projects/garnet/libs/server/Resp/RespServerSession.cs:line 368    
at Garnet.server.RespServerSession.ProcessMessages() in /Users/argsno/Projects/garnet/libs/server/Resp/RespServerSession.cs:line 273    
at Garnet.server.RespServerSession.TryConsumeMessages(Byte* reqBuffer, Int32 bytesReceived) in /Users/argsno/Projects/garnet/libs/server/Resp/RespServerSession.cs:line 199

[badrishc]

Great find and fix, thanks!