Faster version of has_claw function

For a complete graph on 40 vertices:

Old version
```
sage: %time has_claw(g)
CPU times: user 125 ms, sys: 11.4 ms, total: 136 ms
Wall time: 128 ms
False
```

New version
```
sage: %time has_claw(g)
CPU times: user 14.7 ms, sys: 1.34 ms, total: 16 ms
Wall time: 15.4 ms
False
```