Fix mistaken non-calls to exit()

[PhilMiller]

In C, exit; is not an effective statement - it just evaluates the pointer to the function exit(), rather than calling it. This was probably a relic of porting code from Fortran. I noticed a compiler warning about this.

Changes

• Actually call exit() instead of just referencing it

Testing

This is in error handling code, which is not tested (AFAIK)

Screenshots

[  7%] Building C object /repos/ngen/extern/evapotranspiration/evapotranspiration/cmake_build/CMakeFiles/petbmi.dir/src/pet.c.o
/repos/ngen/extern/evapotranspiration/evapotranspiration/src/pet.c:433:9: warning: expression result unused [-Wunused-value]
        exit;
        ^~~~
/repos/ngen/extern/evapotranspiration/evapotranspiration/src/pet.c:462:9: warning: expression result unused [-Wunused-value]
        exit;
        ^~~~

Checklist

• [x] PR has an informative and human-readable title
• [x] Changes are limited to a single goal (no scope creep)
• [x] Code can be automatically merged (no conflicts)
• [x] Code follows project standards (link if applicable)
• [x] Passes all existing automated tests
• [x] Any change in functionality is tested
• [x] New functions are documented (with a description, list of inputs, and expected output)
• [x] Placeholder code is flagged / future todos are captured in comments
• [x] Visually tested in supported browsers and devices (see checklist below :point_down:)
• [x] Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
• [x] Reviewers requested with the Reviewers tool :arrow_right:

[PhilMiller]

@aaraney @madMatchstick Could one of you please review and merge this? I can't add reviewers myself