fixes the memory corruption in #252

Description:

Moved the check and bail out for nfiles=0 to before the arrays with nfiles are used.

Note I have not addressed the question of whether perfect_model_obs should continue (or not) with empty " " . This bug fix just stops the code having a memory error (this error was preventing perfect_model_obs from exiting properly on mac).

Fixes issue

Fixes: #252

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Documentation changes needed?

• My change requires a change to the documentation.
  • I have updated the documentation accordingly.

Tests

Please describe any tests you ran to verify your changes.

Ran lorenz_96 perfect_model_obs with the following namelist option:

output_state_files         = ""

Checklist for merging

• Updated changelog entry
• Documentation updated
• Version tag

Testing Datasets

• Dataset needed for testing available upon request
• Dataset download instructions included
• No dataset needed