Fix bugs #20493, #20494.

Fix bugs #20493, #20494.
* flip.c (struct flip_pgm): Remove `case_size' member (now
unused).
(cmd_flip): Pass var_cnt as number of cases instead of case_cnt,
to fix bug #20494.  Don't assign to `case_size' member, which was
unused after assignment.
(build_dictionary): When NEWNAMES not used, get the number of
variables right, to fix bug #20493.
* tests/command/flip.sh: Add tests to avoid regression on bugs
#20493, #20494.