handle_flux_at_edge: ensure footprint is not over-sized

The 'tfoot' and 'fpcopy' had different sizes, which causes
memory problems in buildSymmetricTemplate.
buildSymmetricTemplate: guard against over-sized footprints

buildSymmetricTemplate now throws exceptions if the footprint
is too large for the image (and would therefore cause memory
problems).
Merge branch 'tickets/HSC-1004'