Fix callback with MINLP and hidden variables

[odow]

Fixes https://discourse.julialang.org/t/memory-corruption-error-in-julia-incorrect-checksum-for-freed-object-when-running-a-jump-gurobi-model/130522

Nonlinear expressions introduce "hidden" variables, so the vector we were allocating for the callback solution was not long enough, and we were overwriting our memory space.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.30%. Comparing base (a04e3f2) to head (9a25d46).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #640   +/-   ##
=======================================
  Coverage   94.30%   94.30%           
=======================================
  Files           6        6           
  Lines        2739     2741    +2     
=======================================
+ Hits         2583     2585    +2     
  Misses        156      156           

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