Fix possible buffer overrun.

The patch to b43 by Michael Buesch <mb@bu3sch.de> has been ported to
b43legacy.
Signed-off-by: NStefano Brivio <stefano.brivio@polimi.it>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

Signed-off-by: NLarry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>