fix error like "offset 0x1f85: partial file", the root cause is when
read the mp4 file from http, and the moov in the end of the mp4 file,
reconfig the buffer will drop some data.
Signed-off-by: Jun Zhao <barryjzhao@tencent.com>
Signed-off-by: Zhongxing Jiang <tomajsjiang@tencent.com>