Reduce potential for nontermination in Derivative

[HarrisonGrodin]

Re: #82.

[codecov]

Codecov Report

Merging #84 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #84      +/-   ##
==========================================
- Coverage    89.7%   89.68%   -0.03%     
==========================================
  Files          11       11              
  Lines         408      407       -1     
==========================================
- Hits          366      365       -1     
  Misses         42       42

Impacted Files	Coverage Δ
src/differentials.jl	93.02% <100%> (-0.16%)	⬇️

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[HarrisonGrodin]

Also, do we want to expose the Derivative function, or should it be kept internal (only for method extension)?

[ChrisRackauckas]

Also, do we want to expose the Derivative function, or should it be kept internal (only for method extension)?

It should probably just be kept as an internal detail.