Add assertions to IConv converter

[Flamefire]

Assert post-conditions of calling iconv and add test for a possible violation of that.

[codecov[bot]]

Codecov Report

Merging #208 (1de61bf) into develop (8445dce) will increase coverage by 0.02%. The diff coverage is 100.00%.

Additional details and impacted files

[](https://app.codecov.io/gh/boostorg/locale/pull/208?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg) ```diff @@ Coverage Diff @@ ## develop #208 +/- ## =========================================== + Coverage 95.63% 95.65% +0.02% =========================================== Files 114 114 Lines 10138 10145 +7 =========================================== + Hits 9695 9704 +9 + Misses 443 441 -2 ``` | [Files](https://app.codecov.io/gh/boostorg/locale/pull/208?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg) | Coverage Δ | | |---|---|---| | [src/boost/locale/encoding/iconv\_converter.hpp](https://app.codecov.io/gh/boostorg/locale/pull/208?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg#diff-c3JjL2Jvb3N0L2xvY2FsZS9lbmNvZGluZy9pY29udl9jb252ZXJ0ZXIuaHBw) | `94.64% <100.00%> (+0.30%)` | :arrow_up: | | [test/test\_encoding.cpp](https://app.codecov.io/gh/boostorg/locale/pull/208?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg#diff-dGVzdC90ZXN0X2VuY29kaW5nLmNwcA==) | `100.00% <100.00%> (ø)` | | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/boostorg/locale/pull/208/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg) ------ [Continue to review full report in Codecov by Sentry](https://app.codecov.io/gh/boostorg/locale/pull/208?src=pr&el=continue&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg). > **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg) > `Δ = absolute (impact)`, `ø = not affected`, `? = missing data` > Powered by [Codecov](https://app.codecov.io/gh/boostorg/locale/pull/208?src=pr&el=footer&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg). Last update [8445dce...1de61bf](https://app.codecov.io/gh/boostorg/locale/pull/208?src=pr&el=lastupdated&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg).