avdevice/xcbgrab: Check sscanf() return

Alot more input checking can be performed, this is only checking the obvious missing case

Fixes: CID1598562 Unchecked return value

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 78d6d13bab)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavdevice/xcbgrab.c

@@ -826,7 +826,10 @@ static av_cold int xcbgrab_read_header(AVFormatContext *s)
 
     if (!sscanf(s->url, "%[^+]+%d,%d", display_name, &c->x, &c->y)) {
         *display_name = 0;
-        sscanf(s->url, "+%d,%d", &c->x, &c->y);
+        if(sscanf(s->url, "+%d,%d", &c->x, &c->y) != 2) {
+            if (*s->url)
+                av_log(s, AV_LOG_WARNING, "Ambigous URL: %s\n", s->url);
+        }
     }
 
     c->conn = xcb_connect(display_name[0] ? display_name : NULL, &screen_num);