Improve robustness of cam open/close file register code to fix issues with history output (#333)

Tag name (required for release branches):
Originator(s): @jimmielin 

Description (include the issue title, and the keyword ['closes',
'fixes', 'resolves'] followed by the issue number):

- Fixes #332 (`max_mdims` used before defined)
- Fixes #331 (unassociated `of%file_desc` in `cam_register_open_file`
leading to crash with >2 history files)

Describe any changes made to build system: N/A

Describe any changes made to the namelist: N/A

List any changes to the defaults for the input datasets (e.g. boundary
datasets): N/A

List all files eliminated and why: N/A

List all files added and what they do: N/A

List all existing files that have been modified, and describe the
changes:
(Helpful git command: `git diff --name-status
development...<your_branch_name>`)

```
Fixes #331
M       src/history/cam_hist_file.F90

Fixes #332
M       src/utils/cam_abortutils.F90

```

If there are new failures (compared to the
`test/existing-test-failures.txt` file),
have them OK'd by the gatekeeper, note them here, and add them to the
file.
If there are baseline differences, include the test and the reason for
the
diff. What is the nature of the change? Roundoff?

derecho/intel/aux_sima:

derecho/gnu/aux_sima:

If this changes climate describe any run(s) done to evaluate the new
climate in enough detail that it(they) could be reproduced:

CAM-SIMA date used for the baseline comparison tests if different than
latest:

Author: jimmielin

src/history/cam_hist_file.F90

@@ -1015,6 +1015,7 @@ subroutine config_define_file(this, restart, logname, host, model_doi_url)
          end do
       end do
       ! Determine the maximum number of dimensions
+      max_mdims = 0
       do field_index = 1, size(this%field_list)
          max_mdims = max(max_mdims, size(this%field_list(field_index)%dimensions()))
       end do

src/utils/cam_abortutils.F90

@@ -80,25 +80,31 @@ subroutine cam_register_open_file(file, file_name)
       end do
       ! If we get here, go ahead and register the file
       if (associated(open_files_pool)) then
+         ! Reuse pooled structure and point to the next pool entry
          of_new => open_files_pool
+         open_files_pool => open_files_pool%next
          allocate(of_new%file_desc, stat=ierr)
          call check_allocate(ierr, subname, 'of_file%file_desc', file=__FILE__, &
              line=__LINE__)
          of_new%file_desc = file
          of_new%file_name = file_name
-         allocate(open_files_pool%next)
-         open_files_pool%next => open_files_pool
+         nullify(of_new%next)
       else
          allocate(of_new)
          allocate(of_new%file_desc)
          of_new%file_desc = file
          of_new%file_name = file_name
-         open_files_pool => of_new
-      end if
-      open_files_tail => of_new
-      if (.not. associated(open_files_head)) then
-         open_files_head => of_new
+         nullify(of_new%next)
       end if
+
+      ! Add the registered file to the tail of the open files list
+      if(associated(open_files_tail)) then
+         open_files_tail%next => of_new
+         open_files_tail      => of_new
+      else
+         open_files_head      => of_new
+         open_files_tail      => of_new
+      endif
    end subroutine cam_register_open_file
 
    subroutine cam_register_close_file(file, log_shutdown_in)