PR Summary

Changes coeff computation in LogFormatter.__call__, LogFormatterMathtext.__call__, and LogFormatterSciNotation to avoid underflow/overflow and adds a simple test.

Closes #18800.

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• [N/A] New features are documented, with examples if plot related.
• [N/A] Documentation is sphinx and numpydoc compliant (the docs should build without error).
• [N/A] Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• [N/A] New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• [N/A] API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).