Bug fix for corner detection with tripolar grids

[aekiss]

This PR consists of @russfiedler's fixes to the logic for detecting corners in tripolar grids when the corners are located at the tripole.

The previous faulty logic caused ESMF_RegridWeightGen (in ESMF 8.0.1) to fail when generating conservative remapping weights with tripolar grids at particular core counts.
Relevant PET*.Log files contained

20200817 221425.005 ERROR            PET270 ESMF_Grid.F90:5303 ESMF_GridCreate Wrong argument specified  - - Bad corner array in SCRIP file
20200817 221425.005 ERROR            PET270 ESMF_Grid.F90:6800 ESMF_GridCreateFrmScrip Wrong argument specified  - Internal subroutine call returned Error
20200817 221425.005 ERROR            PET270 ESMF_Grid.F90:6395 ESMF_GridCreateFrmNCFile Wrong argument specified  - Internal subroutine call returned Error
20200817 221425.005 ERROR            PET270 ESMF_RegridWeightGen.F90:1345 ESMF_RegridWeightGenFile Wrong argument specified  - Internal subroutine call returned Error

The code changes in this PR fix this issue.

For more details see COSIMA/access-om2#216 (comment)