staging: rtl8188eu: prevent an underflow in rtw_check_beacon_data()

BugLink: http://bugs.launchpad.net/bugs/1702104

commit 784047eb upstream.

The "len" could be as low as -14 so we should check for negatives.

Fixes: 9a7fe54d ("staging: r8188eu: Add source files for new driver - part 1")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>

drivers/staging/rtl8188eu/core/rtw_ap.c

@@ -894,7 +894,7 @@ int rtw_check_beacon_data(struct adapter *padapter, u8 *pbuf,  int len)
 		return _FAIL;
 
 
-	if (len > MAX_IE_SZ)
+	if (len < 0 || len > MAX_IE_SZ)
 		return _FAIL;
 
 	pbss_network->IELength = len;