fix bounds check in get_diag_buffer

[rem1776]

Description
fixes the bounds check and replaces the module level variable with this.

Part of the reason I wrote it this way is because fmsDiagObject_type is public so there's nothing guaranteeing that this routine will only be called on fms_diag_object (the module's instance) and not any other new instances of the type.

I guess it's really just a scoping issue but we could potentially add checks that this matches up memory address-wise to fms_diag_object to avoid this or change some public/private declarations.

How Has This Been Tested?
oneapi 2023.0 on amd

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