Fix bug in adaptive time-stepping

[wilfonba]

Description

Fixes a bug where the adaptive time step small dt check failed to trigger due to variable scope.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Scope

• This PR comprises a set of related changes with a common goal

How Has This Been Tested?

Ran a case that triggers this error

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 54.53%. Comparing base (563bc7f) to head (815e51c).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/simulation/m_start_up.fpp	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #667      +/-   ##
==========================================
- Coverage   54.54%   54.53%   -0.01%     
==========================================
  Files          61       61              
  Lines       13800    13802       +2     
  Branches     1727     1727              
==========================================
  Hits         7527     7527              
- Misses       5817     5819       +2     
  Partials      456      456              

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