Correct ellint_1 logic for types with no infinity.

[jzmaddock]

Fixes multiprecision failures.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.71%. Comparing base (435d103) to head (3dafc07).

Additional details and impacted files

[](https://app.codecov.io/gh/boostorg/math/pull/1130?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg) ```diff @@ Coverage Diff @@ ## develop #1130 +/- ## ======================================== Coverage 93.71% 93.71% ======================================== Files 771 771 Lines 61103 61105 +2 ======================================== + Hits 57264 57266 +2 Misses 3839 3839 ``` | [Files](https://app.codecov.io/gh/boostorg/math/pull/1130?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg) | Coverage Δ | | |---|---|---| | [include/boost/math/special\_functions/ellint\_1.hpp](https://app.codecov.io/gh/boostorg/math/pull/1130?src=pr&el=tree&filepath=include%2Fboost%2Fmath%2Fspecial_functions%2Fellint_1.hpp&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg#diff-aW5jbHVkZS9ib29zdC9tYXRoL3NwZWNpYWxfZnVuY3Rpb25zL2VsbGludF8xLmhwcA==) | `100.00% <100.00%> (ø)` | | ------ [Continue to review full report in Codecov by Sentry](https://app.codecov.io/gh/boostorg/math/pull/1130?dropdown=coverage&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/math/pull/1130?dropdown=coverage&src=pr&el=footer&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=boostorg). Last update [435d103...3dafc07](https://app.codecov.io/gh/boostorg/math/pull/1130?dropdown=coverage&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).