b43legacy: fix possible buffer overrun in debugfs

Fix possible buffer overrun.

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

drivers/net/wireless/b43legacy/debugfs.c

@@ -124,7 +124,7 @@ static ssize_t shm_read_file(struct b43legacy_wldev *dev, char *buf, size_t bufs
 	__le16 *le16buf = (__le16 *)buf;
 
 	for (i = 0; i < 0x1000; i++) {
-		if (bufsize <= 0)
+		if (bufsize < sizeof(tmp))
 			break;
 		tmp = b43legacy_shm_read16(dev, B43legacy_SHM_SHARED, 2 * i);
 		le16buf[i] = cpu_to_le16(tmp);