avutil/tx: Use proper deallocator
May fix the FATE failures on x64 Windows here:
https://fate.ffmpeg.org/report.cgi?slot=x86_64-msvc17-windows-native&time=20221125130443
Reviewed-by: Lynne <dev@lynne.ee>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>