pcmcia: fix read buffer overflow

If count > 0 and dev->rlen == dev->rpos and dev->proto == 0 then we read
and write dev->rbuf[-1];
Signed-off-by: NRoel Kluin <roel.kluin@gmail.com>
Cc: Harald Welte <laforge@gnumonks.org>
Cc: Dominik Brodowski <linux@dominikbrodowski.net>
Cc: Greg KH <greg@kroah.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/char/pcmcia/cm4000_cs.c

@@ -1017,7 +1017,7 @@ static ssize_t cmm_read(struct file *filp, __user char *buf, size_t count,
 		}
 	}
 
-	if (dev->proto == 0 && count > dev->rlen - dev->rpos) {
+	if (dev->proto == 0 && count > dev->rlen - dev->rpos && i) {
 		DEBUGP(4, dev, "T=0 and count > buffer\n");
 		dev->rbuf[i] = dev->rbuf[i - 1];
 		dev->rbuf[i - 1] = dev->procbyte;