shared/gatt: Fix incorrect data read
shared/gatt: Fix incorrect data read

Data set in Find By Type Value response contains 4 octets,
2 for start handle and two for end group handle. Reading data
with offset 6 from end of pdu can cause illegal access.

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