avformat/avio: make the logic simple

remove the "ret" to make the code simple and generic.
Signed-off-by: Jun Zhao <mypopydev@gmail.com>

libavformat/avio.c

@@ -663,8 +663,7 @@ int ffurl_shutdown(URLContext *h, int flags)
 
 int ff_check_interrupt(AVIOInterruptCB *cb)
 {
-    int ret;
-    if (cb && cb->callback && (ret = cb->callback(cb->opaque)))
-        return ret;
+    if (cb && cb->callback)
+        return cb->callback(cb->opaque);
     return 0;
 }