vpx_video_(read|writer)_open: fix FILE leaks on error

This adds missing `fclose()`s in failure paths in
`vpx_video_reader_open()` and `vpx_video_writer_open()`. Fixes some
clang-19 static analysis warnings.

Bug: 474642915
Change-Id: Iabe3d5f658e8724692d0b0d55d2ca7f9978b5cbb