Fix bug in Fortran dataset destructor

[ashao]

Fixes a bug in the Fortran client where (on compilers that support automatic destruction) the program would segfault if the user had manually destroyed the dataset. This behaviour was tracked down to an error in the C-client where only the one-level de-referenced pointer was being set to NULL but not the actual double-pointer.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.63%. Comparing base (4937fbd) to head (0a12086).
⚠️ Report is 20 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff              @@
##           develop     #525       +/-   ##
============================================
- Coverage    95.92%   57.63%   -38.30%     
============================================
  Files           10      147      +137     
  Lines          884    11908    +11024     
  Branches         0     1131     +1131     
============================================
+ Hits           848     6863     +6015     
- Misses          36     4894     +4858     
- Partials         0      151      +151     

Files with missing lines	Coverage Δ
src/c/c_dataset.cpp	62.71% <ø> (ø)
src/fortran/dataset.F90	92.30% <100.00%> (ø)
tests/fortran/client_test_dataset.F90	100.00% <100.00%> (ø)

... and 134 files with indirect coverage changes

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[al-rigazzi]

One minor typo in the changelog, but approving as it looks good otherwise. Thanks for fixing this user issue!