tools/l2cap-tester: Fix checking for read() return values
tools/l2cap-tester: Fix checking for read() return values

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