fsm: Correct handling of `fsm_trim` return value.
fsm: Correct handling of `fsm_trim` return value.

Unlike the other functions called there, it returns -1 on error,
or otherwise the number of states returned.

This was caught by `tests/trim/in4.fsm`, which isn't expected to
remove any states, but its 0 result was treated as false, meaning
it had failed.

katef/libfsm,katef/libfsm,katef/libfsm,katef/libfsm,katef/libfsm