Bluetooth: host: Prevent race condition in pairing procedure

Moving the key check after checking that no pairing or encryption
procedure is started fixes a race condition that is seen in some PTS
tests:

GATT/CL/GAR/BI-04-C
GATT/CL/GAR/BI-16-C
GATT/CL/GAR/BI-21-C
GATT/CL/GAW/BI-05-C
GATT/CL/GAW/BI-12-C

Signed-off-by: Jonathan Rico <jonathan.rico@nordicsemi.no>