demmt: fix decoding of 0-size long commands
demmt: fix decoding of 0-size long commands

Not seen in any trace. Found by code review.

kfractal/envytools,grate-driver/envytools,karolherbst/envytools,pierremoreau/envytools,karolherbst/envytools,grate-driver/envytools,hakzsam/envytools,MoochMcGee/envytools,envytools/envytools,karolherbst/envytools,envytools/envytools,pierremoreau/envytools,kfractal/envytools,hakzsam/envytools,hakzsam/envytools,envytools/envytools,hakzsam/envytools,grate-driver/envytools,kfractal/envytools,envytools/envytools,envytools/envytools,pierremoreau/envytools,pierremoreau/envytools,MoochMcGee/envytools,MoochMcGee/envytools,MoochMcGee/envytools,grate-driver/envytools,kfractal/envytools,karolherbst/envytools,pierremoreau/envytools,kfractal/envytools,hakzsam/envytools,grate-driver/envytools,MoochMcGee/envytools,karolherbst/envytools