maint: add assertions to placate static analysis tools

maint: add assertions to placate static analysis tools

A static analysis tool (http://labs.oracle.com/projects/parfait/)
produced some false positive diagnostics.  Add assertions to help
it understand that the code is correct.
* src/stty.c: Include <assert.h>.
(display_changed): Add an assertion to placate parfait.
(display_all): Likewise.
* src/sort.c: Include <assert.h>.
(main): Add an assertion to placate parfait.
* src/fmt.c: Include <assert.h>.
(get_paragraph): Add an assertion to placate parfait.