Follow-up from "libs: audio: Fix gst_audio_buffer_truncate meta handling"

The following discussions from !1035 (merged) should be addressed:

• @slomo started a discussion:

  Oh good, this is also wrong. If trim==0 then it will keep duration, pts, etc. and it would be wrong. If trim!=0 then it would replace them all with GST_CLOCK_TIME_NONE, which is not wrong but also not great.

• @slomo started a discussion: (+2 comments)

  ... and in this branch it uses gst_buffer_make_writable() so would always preserve the old pts, duration, etc. and those would usually be wrong unless trim==0 and samples==orig_samples.

  So there's a bit more to fix here, but your change is definitely correct :)

  Also this wouldn't have compiled in Rust