Fix API failure w/ 4 atoms

[godotalgorithm]

Fixes #230. This bug was introduced during the transition between the commercial and open-source versions of MOPAC at commit 93c7072. The most likely explanation is that it was meant to be exit to leave a conditional statement (although the statement appeared inside a larger if block rather than a do loop and would not have worked as such), rather than call exit(1), which is a GNU Fortran extension to terminate the program (that is otherwise not used anywhere else in MOPAC).

Status

• Ready for merge

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.98%. Comparing base (48500c0) to head (820ebdb).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #232   +/-   ##
=======================================
  Coverage   67.98%   67.98%           
=======================================
  Files         339      339           
  Lines       77435    77434    -1     
=======================================
+ Hits        52646    52647    +1     
+ Misses      24789    24787    -2     

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