Prevent signed integer overflow in maxGridDiskSize

[isaacbrodsky]

Signed integer overflow is undefined behavior in C, so we cannot add, multiply, etc. arbitrary integer inputs. This adds a hard cap on the maxGridDiskSize function based on knowledge of the grid system itself.

[coveralls]

Coverage increased (+0.003%) to 99.038% when pulling a0c4bbe5a97741cec0813687071518fcf21fa46f on isaacbrodsky:prevent-signed-integer-overflow-maxgriddisksize into f581626150e2e2b02f0ee1636084dfb8a4563abf on uber:master.