Fix bugs, compiler warnings, and test includes from commit review

[IAKOBVS]

Reviewed the 2 latest commits (8379140 and e66b9f5) and fixed identified issues:

1. Fixed potential buffer overflow in jstr_repeat_len (include/replace.h) by checking for n > (SIZE_MAX - 1) / *sz and handling *sz == 0.
2. Resolved -Wunused-variable and -Wunused-function compiler warnings in tests/test-io-extra5.c and tests/test-io-ftw2.c.
3. Fixed build error in tests/test-nosimd.c by updating header path to "jstr.h".
4. Recompiled and verified all 126 test variants pass with 0 failures.

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PR created automatically by Jules for task 1981864023977431433 started by @IAKOBVS

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