Remove a ;, to allow Rust to infer the type (RustCrypto#1374)

`?` currently influences inference s.t. for `error()?` rustc infers `T = ()`.
However, it is quite confusing -- `?` is not supposed to influence inference,
it's just a conicidence because of the curent implementation. There is an idea
to change this behavior in future Rust versions, such that this code won't
compile without this change.

der/src/writer/slice.rs

@@ -31,7 +31,7 @@ impl<'a> SliceWriter<'a> {
     /// Encode a value which impls the [`Encode`] trait.
     pub fn encode<T: Encode>(&mut self, encodable: &T) -> Result<()> {
         if self.is_failed() {
-            self.error(ErrorKind::Failed)?;
+            self.error(ErrorKind::Failed)?
         }
 
         encodable.encode(self).map_err(|e| {