tools/gatt: Fix not checking malloc() failure
tools/gatt: Fix not checking malloc() failure

This follows standard practice elsewhere in this file.

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