RSN: Do not start preauthentication timer without candidates

RSN: Do not start preauthentication timer without candidates

There is no need to schedule the postponed RSN preauthentication start
if there are no candidates. Avoid wasting eloop resources for this.

This is most useful for fuzz testing of the 4-way handshake
implementation to avoid getting stuck waiting for this unnecessary one
second time when using eloop to coordinate the Authenticator and
Supplicant state machines.

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