Fix authenticator OKC fetch from PMKSA cache to avoid infinite loop

Fix authenticator OKC fetch from PMKSA cache to avoid infinite loop

If the first entry in the PMKSA cache did not match the station's MAC
address, an infinite loop could be reached in pmksa_cache_get_okc() when
trying to find a PMKSA cache entry for opportunistic key caching cases.
This would only happen if OKC is enabled (okc=1 included in the
configuration file).

Signed-off-by: Jouni Malinen <j@w1.fi>