tac: remove MMAP use of untrusted source #9748

[cerdelen]

truncation of the mapped region resulted in core dumped.

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/printf/printf-surprise is now passing!
Note: The gnu test tests/dd/no-allocate was skipped on 'main' but is now failing.
Note: The gnu test tests/misc/write-errors was skipped on 'main' but is now failing.

[cerdelen]

fixes #9748

[codspeed-hq]

CodSpeed Performance Report

Merging #9772 will not alter performance

_{Comparing cerdelen:main (6b3abce) with main (2d718f2)}

Summary

✅ 127 untouched
⏩ 6 skipped¹

Footnotes

1. 6 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/printf/printf-surprise is now passing!
Note: The gnu test tests/dd/no-allocate was skipped on 'main' but is now failing.
Note: The gnu test tests/misc/write-errors was skipped on 'main' but is now failing.

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/printf/printf-surprise is now passing!
Note: The gnu test tests/dd/no-allocate was skipped on 'main' but is now failing.
Note: The gnu test tests/misc/write-errors was skipped on 'main' but is now failing.