Fix one of two errors on edge

This fixes 70~ errors on edge Rust. However, there's one last error I can't quite figure out. If you know how you want to fix it, I can add another commit here.

```
   Compiling protobuf v0.0.2 (file:///home/steve/src/rust-protobuf)
src/lib/unknown.rs:100:58: 100:74 error: `length_delimited` does not live long enough
src/lib/unknown.rs:100             return Some(UnknownValueRef::LengthDelimited(length_delimited.as_ref().unwrap().as_slice()))
                                                                                ^~~~~~~~~~~~~~~~
src/lib/unknown.rs:85:55: 103:6 note: reference must be valid for the lifetime 'o as defined on the block at 85:54...
src/lib/unknown.rs:85     fn next(&mut self) -> Option<UnknownValueRef<'o>> {
src/lib/unknown.rs:86         let fixed32 = self.fixed32.next();
src/lib/unknown.rs:87         if fixed32.is_some() {
src/lib/unknown.rs:88             return Some(UnknownValueRef::Fixed32(*fixed32.unwrap()));
src/lib/unknown.rs:89         }
src/lib/unknown.rs:90         let fixed64 = self.fixed64.next();
                      ...
src/lib/unknown.rs:85:55: 103:6 note: ...but borrowed value is only valid for the block at 85:54
src/lib/unknown.rs:85     fn next(&mut self) -> Option<UnknownValueRef<'o>> {
src/lib/unknown.rs:86         let fixed32 = self.fixed32.next();
src/lib/unknown.rs:87         if fixed32.is_some() {
src/lib/unknown.rs:88             return Some(UnknownValueRef::Fixed32(*fixed32.unwrap()));
src/lib/unknown.rs:89         }
src/lib/unknown.rs:90         let fixed64 = self.fixed64.next();
                      ...

```