avformat/movenchint: Simplify writing padding

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavformat/movenchint.c

@@ -260,8 +260,7 @@ static void output_immediate(const uint8_t *data, int size,
         data += len;
         size -= len;
 
-        for (; len < 14; len++)
-            avio_w8(out, 0);
+        ffio_fill(out, 0, 14 - len);
 
         (*entries)++;
     }