microchip-pickit2: silence compiler warnings (NULL dereference)

microchip-pickit2: silence compiler warnings (NULL dereference)

Check pointers for validity before dereferencing them. This silences the
following compiler warnings:

../src/hardware/microchip-pickit2/api.c:275:8: warning: Access to field 'conn' results in a dereference of a null pointer (loaded from variable 'sdi')
                if (!sdi->conn)
../src/hardware/microchip-pickit2/api.c:281:10: warning: Access to field 'samplerates' results in a dereference of a null pointer (loaded from variable 'devc')
                rate = devc->samplerates[devc->curr_samplerate_idx];
../src/hardware/microchip-pickit2/api.c:287:11: warning: Access to field 'captureratios' results in a dereference of a null pointer (loaded from variable 'devc')
                ratio = devc->captureratios[devc->curr_captureratio_idx];