Fix mpp_gather/scatter memory leak

[abrooks1085]

Description
This PR fixes Valgrind-reported memory errors in the MPP gather/scatter paths.

The issue was caused by passing either an unassociated pointer array or an unallocated allocatable array as an actual argument to a subroutine whose corresponding dummy argument is an assumed-shape intent(in) array. In these cases, the call is invalid because the assumed-shape dummy requires a valid array descriptor from the actual argument. Even when the data is only logically used on the root rank, forwarding an invalid array argument on another rank can still expose undefined descriptor state and trigger memory diagnostics.

This PR updates the affected gather/scatter wrapper paths so that all ranks pass valid array arguments into the lower-level routines. For ranks that do not logically contribute or consume the full root-side buffer, a valid temporary array/view is used instead of forwarding an unassociated pointer or unallocated allocatable.

Fixes #1890

How Has This Been Tested?

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included
• make distcheck passes