Protect merge of meltsliq when tr_snow is off in merge_fluxes

[apcraig]

PR checklist

• Short (1 sentence) summary of your PR:
  Avoid computations on uninitialized variable meltsliq in merge_fluxes
• Developer(s):
  apcraig, DeniseWorthen
• Suggest PR reviewers from list in the column to the right.
• Please copy the PR test results link or provide a summary of testing completed below.
  Tested on cheyenne with intel, results as expected, https://github.com/CICE-Consortium/Test-Results/wiki/icepack_by_hash_forks#8da18e20e7a278f8f70f92d50c5d4f7b1b1cc478
• How much do the PR code changes differ from the unmodified code?
  • bit for bit
  • different at roundoff level
  • more substantial
• Does this PR create or have dependencies on CICE or any other models?
  • Yes
  • No
• Does this PR add any new test cases?
  • Yes
  • No
• Is the documentation being updated? ("Documentation" includes information on the wiki or in the .rst files from doc/source/, which are used to create the online technical docs at https://readthedocs.org/projects/cice-consortium-cice/.)
  • Yes
  • No, does the documentation need to be updated at a later time?
    • Yes
    • No
• Please provide any additional information or relevant details below:

Fixes a problems that arises in debug mode when initializing data to Nans. Similar to iso implementation in same routine.