Fix CVE-2016-7953:

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-7953

Patch copied from upstream source repository:

https://cgit.freedesktop.org/xorg/lib/libXvMC/commit/?id=2cd95e7da8367cccdcdd5c9b160012d1dec5cbdb

From 2cd95e7da8367cccdcdd5c9b160012d1dec5cbdb Mon Sep 17 00:00:00 2001
From: Tobias Stoeckmann <tobias@stoeckmann.org>
Date: Sun, 25 Sep 2016 22:34:27 +0200
Subject: [PATCH] Avoid buffer underflow on empty strings.

If an empty string is received from an x-server, do not underrun the
buffer by accessing "rep.nameLen - 1" unconditionally, which could end
up being -1.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>
Reviewed-by: Matthieu Herrb <matthieu@herrb.eu>
---
 src/XvMC.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/XvMC.c b/src/XvMC.c
index 7336760..3ee4212 100644
--- a/src/XvMC.c
+++ b/src/XvMC.c
@@ -576,9 +576,9 @@ Status XvMCGetDRInfo(Display *dpy, XvPortID port,
 	if (*name && *busID && tmpBuf) {
 	    _XRead(dpy, tmpBuf, realSize);
 	    strncpy(*name,tmpBuf,rep.nameLen);
-	    (*name)[rep.nameLen - 1] = '\0';
+	    (*name)[rep.nameLen == 0 ? 0 : rep.nameLen - 1] = '\0';
 	    strncpy(*busID,tmpBuf+rep.nameLen,rep.busIDLen);
-	    (*busID)[rep.busIDLen - 1] = '\0';
+	    (*busID)[rep.busIDLen == 0 ? 0 : rep.busIDLen - 1] = '\0';
 	    XFree(tmpBuf);
 	} else {
 	    XFree(*name);
-- 
2.10.1

