EAP-SIM/AKA: Do not allow anonymous@realm "pseudonym" to be cleared

EAP-SIM/AKA: Do not allow anonymous@realm "pseudonym" to be cleared

If the EAP-SIM/AKA server does not provide a new pseudonym and the
locally configured "pseudonym" in anonymous_identity is actually an
anonymous identitity instead of a real EAP-SIM/AKA pseudonym, do not
clear the anonymous_identity network profile parameter. This is needed
to avoid forgetting the anonymous identity when going through
EAP-SIM/AKA authentication and then reverting back to using IMSI-based
(e.g., encrypted) identity.

Fixes: 4df4133917ab ("EAP-SIM/AKA: Add support for anonymous@realm")
Signed-off-by: Jouni Malinen <jouni@codeaurora.org>