Fixed oops if an uninitialized key is used for encryption.

Without this patch, if you try and use a key that has not been
configured, for example:

% iwconfig eth1 key deadbeef00 [2]

without having configured key [1], then the active key will still be
[1], but privacy will now be enabled.  Transmission of a packet in this
situation will result in a kernel oops.

Signed-off-by: James Ketrenos <jketreno@linux.intel.com>