Fix format issue in ffmpeg (#17685)

* Fix format issue in ffmpeg

This fix tries to address the issue raised in 17533 where
some ouput generated by ffmpeg may not be parsed correctly.

There are two issues in the ffmpeg parser:
1. The size of the frame could be `rgb24, 640x360 [SAR 1:1 DAR 16:9]`,
with extra `[...]` after `640x360`.
2. The number of frames could be `frame=12488` or `frame=  166`, with or
without the sapce (` ` in between `frame=` and the number).

This fix fixes the parser issues in ffmpeg.

This fix fixes 17533.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Fix ffmpeg parser for `frame=  166 ` or `frame=12488 `

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>