Fix: depth first visit problem

[MouJieQin]

Thanks for your code, it helps me a lot. It seems there is a problem in depth first visit logic.

The original fuzzy test:

time ../fst fuzzy words.fst ani
Aani: 6
ai: 4318
an: 7442
ani: 8644
anil: 8660
anis: 8745
../fst fuzzy words.fst ani  0.03s user 0.01s system 79% cpu 0.051 total

The fuzzy test after my fix:

time ./cmd/fst fuzzy words.fst ani

Aani: 6
Dani: 48553
Pani: 138204
ai: 4318
an: 7442
ani: 8644
anil: 8660
anis: 8745
anti: 9807
arni: 13398
bani: 18312
mani: 111111
ni: 124291
rani: 163242
./cmd/fst fuzzy words.fst ani  0.03s user 0.01s system 74% cpu 0.049 total