libavdevice/gdigrab: change hwnd tail check fail logic to !=null

Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>

libavdevice/gdigrab.c

@@ -281,7 +281,7 @@ gdigrab_read_header(AVFormatContext *s1)
 
         hwnd = (HWND) strtoull(name, &p, 0);
 
-        if (p == NULL || p == name || p[0] == '\0')
+        if (p == NULL || p == name || p[0] != '\0')
         {
             av_log(s1, AV_LOG_ERROR,
                    "Invalid window handle '%s', must be a valid integer.\n", name);