Fix NEON/CL CustomConvolution failures

Fix NEON/CL CustomConvolution failures

Properly handle UNDEFINED border mode as done in the reference

Change-Id: I23b117be3ee5cd0e566f6161681d38493627fc9b
Resolves: COMPMID-4133
Signed-off-by: Manuel Bottini <manuel.bottini@arm.com>
Reviewed-on: https://review.mlplatform.org/c/ml/ComputeLibrary/+/4919
Reviewed-by: Michele Di Giorgio <michele.digiorgio@arm.com>
Reviewed-by: Giorgio Arena <giorgio.arena@arm.com>
Comments-Addressed: Arm Jenkins <bsgcomp@arm.com>
Tested-by: Arm Jenkins <bsgcomp@arm.com>