stream_ffmpeg.c: change reads back to url_read_complete()

Change reading data from FFmpeg from url_read() back to
url_read_complete(). url_read_complete() behavior has changed in
FFmpeg and no longer returns an error for partial reads at EOF so it's
usable now. Using it is probably not significantly better than
url_read(), but at least it handles EAGAIN.

stream/stream_ffmpeg.c

@@ -27,7 +27,7 @@
 
 static int fill_buffer(stream_t *s, char *buffer, int max_len)
 {
-    int r = url_read(s->priv, buffer, max_len);
+    int r = url_read_complete(s->priv, buffer, max_len);
     return (r <= 0) ? -1 : r;
 }