Add RTC_CHECK to fwrite calls in video_file_writer
The compiler option `warn_unused_result` causes build failures if the return value of `fwrite` is ignored.
Currently, `video_file_writer.cc` ignores these return values, which is also unsafe if the disk is full.

This CL wraps `fwrite` calls in `RTC_CHECK_EQ` to ensure the correct number of bytes are written, crashing safely if the write fails.

Bug: None
Test: Compiled locally.
Change-Id: I6cb62b673af2d3cefc6afdb9064d86bd9f080cd3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/447020
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46859}