tools/btgatt-client: Fix off-by-one error.
tools/btgatt-client: Fix off-by-one error.

This patch fixes an error in write-long-value command that caused an
extra invalid byte to get sent at the end of the long write value.

pstglia/external-bluetooth-bluez,silent-snowman/bluez,pkarasev3/bluez,ComputeCycles/bluez,ComputeCycles/bluez,ComputeCycles/bluez,pstglia/external-bluetooth-bluez,silent-snowman/bluez,pstglia/external-bluetooth-bluez,mapfau/bluez,silent-snowman/bluez,pstglia/external-bluetooth-bluez,silent-snowman/bluez,mapfau/bluez,pkarasev3/bluez,ComputeCycles/bluez,mapfau/bluez,mapfau/bluez,pkarasev3/bluez,pkarasev3/bluez