opensc: fix spurious gcc overflow warning

(see https://github.com/OpenSC/OpenSC/pull/102#issuecomment-11847241 )

On some architectures (ppc & s390) gcc produces these warnings
  pkcs15-cardos.c: In function 'cardos_select_pin_reference':
  pkcs15-cardos.c:177:5: warning: assuming signed overflow does not occur when assuming that (X + c) < X is always false [-Wstrict-overflow]

  pkcs15-incrypto34.c: In function 'incrypto34_select_pin_reference':
  pkcs15-incrypto34.c:196:5: warning: assuming signed overflow does not occur when assuming that (X + c) < X is always false [-Wstrict-overflow]

Remove this warning by reordering test (there is no real bug anyway).

Signed-off-by: Milan Broz <mbroz@redhat.com>