Bluetooth: GATT: Fix discover callback invalid pointer

Fix issues that surfaced when trying out GCC 9.2, official release name
gcc-arm-none-eabi-9-2019-q4-major, both related to invalid pointers in
GATT when declaring UUIDs in if-scope.

1. Fix the discovery callback giving an invalid pointer in the discovery
callback in two instances.
2. Fix gatt_find_type sending invalid data during discovery procedure.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>