Update: Wrap up unit tests for fl_fss_payload_header_map() and fix observed problems.

Update: Wrap up unit tests for fl_fss_payload_header_map() and fix observed problems.

The unit tests for fl_fss_payload_header_map() are lacking significant combinations and permutations.
This, however, is being considered good enough for a first pass.
More complex and broader cases will hopefully be handled in some likely relatively distant future.

The null_quantity flag does not make any sense as the variable "total" can be 0 and that should not logically be considered NULL.
Make sure strings are NULL terminated for the cases where the code adds an extra space and then unassigned the used length of that space.
Handle cases where range.start > range.stop, aka NULL range.
Various other fixes.