Fail parsing routes with only one point

```python
from traffic.core import FlightPlan

fp = FlightPlan('N0435F300 GZO3D GZO M622 ADEXI/N0457F360 DCT ADUKA DCT LULIX DCT EPIKU DCT GINOX UM732 NISEK UT175 DIVKO UM731 FJR DCT GAI DCT SECHE DCT UVELI DCT CNA DCT MANAK UY110 TIRAV UT183 BAGAD DCT DOLUR M142 INSUN N160 LEDGO DCT KURUM KURUM2D')
fp.all_points
```

Reported by @richardalligier