Bug fix fv_regional_bc.F90: initialize cloud amount boundary conditions only if cloud amount is a valid tracer

[DomHeinzeller]

Description

As the title says. In model/fv_regional_bc.F90, initialize cloud amount boundary conditions to zero only if cloud amount is a valid tracer (i.e. if the index in the tracer array is > 0).

Fixes #145

Tested in the latest ufs-weather-model for regression tests that are run only sporadically (rt_ccpp_dev.conf), i.e. not as part of the standard Intel/GNU tests. This solves a crash with old input data that was generated for Thompson MP and doesn't have cloud amount.

Checklist:

Please check all whether they apply or not

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• [n/a] I have commented my code, particularly in hard-to-understand areas
• [n/a] I have made corresponding changes to the documentation
• My changes generate no new warnings
• [n/a] Any dependent changes have been merged and published in downstream modules

[DomHeinzeller]

I don't know what GitHub was doing, but this issue has not been solved yet. Reopening.