Fix a problem with associating to wifi networks with a hidden SSID.

Fix a problem with associating to wifi networks with a hidden SSID.

If an AP is configured to hide its SSID it sends a non-zero length SSID
which contains only zeroes. The AP sends its actual SSID only in probe
responses after a client includes this SSID in a probe request.
If we happened to receive a beacon before the probe response we stored a
non-zero-length SSID of zeroes and never updated the SSID when the probe
response arrived. The client was then unable to find the AP.

test & ok jung@