Fix second order integral regression

[thangleiter]

Limiting cases

• $\omega + \Omega_{mn} = 0 \wedge \Omega_{ij} - \omega \neq 0$
• $\omega + \Omega_{mn} = 0 \wedge \Omega_{ij} - \omega = 0$

of the second-order integral $I_{ijmn}^{(g)}(\omega)$ were broken in 115653b.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.49%. Comparing base (89dc785) to head (f24933c).
⚠️ Report is 3 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #117      +/-   ##
==========================================
+ Coverage   97.46%   97.49%   +0.03%     
==========================================
  Files           9        9              
  Lines        2407     2396      -11     
  Branches      450      450              
==========================================
- Hits         2346     2336      -10     
  Misses         33       33              
+ Partials       28       27       -1     

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