netdoc: Add a workaround for C Tor's lack of mid-layer NL

It turns out that C Tor doesn't add a newline at the end of the
middle layer of an onion service descriptor.  I've made a spec MR
(torspec!109) to document this: here, it's time to work around the
issue.

crates/tor-netdoc/src/doc/hsdesc/middle_layer.rs

@@ -233,8 +233,7 @@ mod test {
             .dangerously_assume_wellsigned()
             .dangerously_assume_timely();
         let subcred = TEST_SUBCREDENTIAL.into();
-        let mut body = desc.decrypt_body(&subcred).unwrap();
-        body.push(b'\n'); // XXXX BUG in C tor: this \n is really supposed to be there!
+        let body = desc.decrypt_body(&subcred).unwrap();
         let body = std::str::from_utf8(&body[..]).unwrap();
 
         let middle = HsDescMiddle::parse(body)?;

crates/tor-netdoc/src/doc/hsdesc/outer_layer.rs

@@ -73,6 +73,11 @@ impl HsDescOuter {
         let mut body = decrypt.decrypt(&self.encrypted_body[..])?;
         let n_padding = body.iter().rev().take_while(|n| **n == 0).count();
         body.truncate(body.len() - n_padding);
+        // Work around a bug in the C tor implementation: it doesn't
+        // NL-terminate the final line of the middle layer.
+        if !body.ends_with(b"\n") {
+            body.push(b'\n');
+        }
         Ok(body)
     }
 }