The attribute uses 'Actual Identity Length' field to indicate the exact
(pre-padding) length of the Identity. This actual length should be used
as the length, not the remaining attribute length.
This was previously worked around by stripping null termination away
from the end of the identity string at EAP-SIM and EAP-AKA server code.
However, it is likely that that workaround is not really needed and the
real problem was in AT_IDENTITY parsing. Anyway, the workaround is left
in just in case it was really needed with some implementations.
This change breaks interoperability with older wpa_supplicant versions
(everything up to and including wpa_supplicant 0.5.10 and 0.6.5) which
incorrectly used this field as number of bytes, not bits, in RES.
