Fix UB on invalid index in format string

[codecov[bot]]

Codecov Report

Merging #158 (d6e6071) into develop (a8e3a23) will increase coverage by 0.07%. The diff coverage is 95.91%.

Additional details and impacted files

[](https://codecov.io/gh/boostorg/locale/pull/158?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg) ```diff @@ Coverage Diff @@ ## develop #158 +/- ## =========================================== + Coverage 89.44% 89.52% +0.07% =========================================== Files 109 109 Lines 9457 9534 +77 =========================================== + Hits 8459 8535 +76 - Misses 998 999 +1 ``` | [Impacted Files](https://codecov.io/gh/boostorg/locale/pull/158?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg) | Coverage Δ | | |---|---|---| | [include/boost/locale/format.hpp](https://codecov.io/gh/boostorg/locale/pull/158?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg#diff-aW5jbHVkZS9ib29zdC9sb2NhbGUvZm9ybWF0LmhwcA==) | `94.32% <ø> (ø)` | | | [src/boost/locale/shared/format.cpp](https://codecov.io/gh/boostorg/locale/pull/158?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg#diff-c3JjL2Jvb3N0L2xvY2FsZS9zaGFyZWQvZm9ybWF0LmNwcA==) | `83.62% <83.33%> (-1.60%)` | :arrow_down: | | [src/boost/locale/std/std\_backend.cpp](https://codecov.io/gh/boostorg/locale/pull/158?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg#diff-c3JjL2Jvb3N0L2xvY2FsZS9zdGQvc3RkX2JhY2tlbmQuY3Bw) | `82.85% <92.59%> (-0.64%)` | :arrow_down: | | [src/boost/locale/util/default\_locale.cpp](https://codecov.io/gh/boostorg/locale/pull/158?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg#diff-c3JjL2Jvb3N0L2xvY2FsZS91dGlsL2RlZmF1bHRfbG9jYWxlLmNwcA==) | `92.59% <100.00%> (ø)` | | | [src/boost/locale/util/numeric.hpp](https://codecov.io/gh/boostorg/locale/pull/158?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg#diff-c3JjL2Jvb3N0L2xvY2FsZS91dGlsL251bWVyaWMuaHBw) | `79.28% <100.00%> (+1.58%)` | :arrow_up: | | [test/test\_formatting.cpp](https://codecov.io/gh/boostorg/locale/pull/158?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg#diff-dGVzdC90ZXN0X2Zvcm1hdHRpbmcuY3Bw) | `99.68% <100.00%> (+<0.01%)` | :arrow_up: | | [test/test\_util.cpp](https://codecov.io/gh/boostorg/locale/pull/158?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg#diff-dGVzdC90ZXN0X3V0aWwuY3Bw) | `100.00% <100.00%> (ø)` | | ... and [3 files with indirect coverage changes](https://codecov.io/gh/boostorg/locale/pull/158/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://codecov.io/gh/boostorg/locale/pull/158?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://codecov.io/gh/boostorg/locale/pull/158?src=pr&el=footer&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg). Last update [a8e3a23...d6e6071](https://codecov.io/gh/boostorg/locale/pull/158?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).