attrib: Fix Read Blob error responses
attrib: Fix Read Blob error responses

Read blob response with offset equal to value length should return
zero length value intead of Invalid Offset Error.
Found during qualification test case TC/GAR/SR/BV-08-C Read behind
Long Characteristic Descriptor.

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