fix(auth): require bit strings to have no unused bits

This is already checked by the crypto module or by mbedTLS, but checking
it in the X.509 parser is harmless.

Change-Id: Ifdbe3b4c6d04481bb8e93106ee04b49a70f50d5d
Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com>