Fixes WAV import being cut in half with new trim code

[theisegeberg]

Fixes
#27925 "Trim" option on importing WAV stereo audio removes half of it

[akien-mga]

Thanks, looks good :)

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[ghost]

Thanks for contributing! Can confirm that this fixes the issue. Although the thumbnail rendering of the result audiostream doesn't update on reimport, it's unrelated to this issue.

[akien-mga]

Thanks!

[akien-mga]

Note to release managers: should go together with #27645.

[akien-mga]

Cherry-picked for 3.1.2.