Ambiguous parse?

The fuzzer returned this test case. Not sure if it's a bug or not, but it's a case where `Unmarshal(x).Marshal() != x`

Unmarshaling this SDP:

```
v=0
o=0 0 0 IN IP4 0
s=
t=0 0
```

Produces this SDP when marshaled:

```
v=0
o= 0 0   
s=
```