lavf/async: Fix ring_write return value

This fixes a regression from commit 36117968ad.

wrapped_url_read() used to be able to return positive number from
ffurl_read(). It relies on the result to check if EOF is reached in
async_buffer_task().

But FIFO callbacks must return 0 on success. This should be handled
in ring_write() instead.

Test case:
  ffmpeg -f lavfi -i testsrc -t 1 test.mp4
  ffmpeg -i async:test.mp4

Signed-off-by: Guangyu Sun <gsun@roblox.com>
Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavformat/async.c

@@ -144,8 +144,14 @@ static int wrapped_url_read(void *src, void *dst, size_t *size)
 
 static int ring_write(RingBuffer *ring, URLContext *h, size_t size)
 {
+    int ret;
+
     av_assert2(size <= ring_space(ring));
-    return av_fifo_write_from_cb(ring->fifo, wrapped_url_read, h, &size);
+    ret = av_fifo_write_from_cb(ring->fifo, wrapped_url_read, h, &size);
+    if (ret < 0)
+        return ret;
+
+    return size;
 }
 
 static int ring_size_of_read_back(RingBuffer *ring)