EAP-FAST: Clean up binary PAC file parser validation steps

EAP-FAST: Clean up binary PAC file parser validation steps

This was too difficult for some static analyzers (CID 62876). In
addition, the pac_info_len assignment should really have explicitly
validated that there is room for the two octet length field instead of
trusting the following validation step to handle both this and the
actual pac_info_len bounds checking.

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