Fix memory leak in tryopen.

[DeNdlerX]

tryopen -s gets the error message with zip_error_strerror() but never calls zip_error_fini(), so the string leaks for every archive that fails to open. When the tests are built with ASan on Linux, this makes open_incons, open_incons_eocd64_gap, open_incons_utf8_surrogate and extra_field_align fail with a LeakSanitizer report.

cmake -B build -DCMAKE_C_FLAGS="-fsanitize=address -g"
cmake --build build
ctest --test-dir build -R 'open_incons|extra_field_align'

On main, 4 of the 5 selected tests fail; with this change all of them pass.

The two other LeakSanitizer failures in the suite (read_incons and decrypt-wrong-password-pkware-2) come from zip_source_free() and are fixed by #591. With both applied, the full test suite passes under ASan with no leaks reported.

If you're interested, I can also add a CI job that runs the tests with -fsanitize=address,undefined on Linux, so issues like these are caught automatically.