[ifconfig] handle illegal WPS frames

Some APs broadcast WPS IE frames with totally broken data. Ifconfig's printwpsie()
loops through WPS frames printing the attributes out; if the frame's data is bad,
printwpsie() can end up looking at out-of-bounds addresses causing ifconfig to
bus error.

Thanks to Takashi Inoue at Nihon U for his efforts in debugging this.

PR: bin/217312
Submitted by: fbsd@opal.com
MFC after: 1 week