filter: compare as integers to make sure we're within the right bounds.

Fixes CID 1189413: Unsigned compared against 0 (NO_EFFECT)
filter: set fft and ifft pointers to NULL not 0 in constructor.

Addresses CID 1189412: Explicit null dereferenced (FORWARD_NULL)
Merge branch 'maint'
filter: completing previous fix for CID 1189412 for new ccf filter on master.