Fix a crash in iwx(4) when connecting to WEP networks via ifconfig join.

Fix a crash in iwx(4) when connecting to WEP networks via ifconfig join.

In if_iwx.c r1.63 / CVS commit ID QzTzJtyPl10L63Yi, I fixed a problem with
connecting to WPA1/TKIP networks. This change introduced a deference of
the 'ni' pointer passed to ic_set_key(). This pointer will be passed in
as NULL in certain situations, such as when WEP is used. Add the missing
NULL check.

reported by and fix tested by matthieu@