Support infinite weights in lt-comp/lt-proc

We need to implement a way to represent infinite weights.
The current outcome is strange!
```
$ cat sample.att
0 1 a b 2
1 2 b c 1
1 2 c d inf
2 0

$ lt-comp lr sample.att sa.bin
main@standard 3 3

$ lt-print sa.bin
0 1 a b 1.000000
1 2 b c 2.000000
1 2 c d -2.000000
2 0.000000
```