Return correct ConnectionTrafficSecrets variant when AES-256-GCM is…

… negotiated.

55bb279 inadvertently changed `extract_keys`
to always return `ConnectionTrafficSecrets::Aes128Gcm`, even when AES-256-GCM
was negotiated. This change fixes it by restoring the key length check.

Fixes #1833

rustls/src/crypto/ring/tls12.rs

@@ -168,9 +168,11 @@ impl Tls12AeadAlgorithm for GcmAlgorithm {
         write_iv: &[u8],
         explicit: &[u8],
     ) -> Result<ConnectionTrafficSecrets, UnsupportedOperationError> {
-        Ok(ConnectionTrafficSecrets::Aes128Gcm {
-            key,
-            iv: gcm_iv(write_iv, explicit),
+        let iv = gcm_iv(write_iv, explicit);
+        Ok(match self.0.key_len() {
+            16 => ConnectionTrafficSecrets::Aes128Gcm { key, iv },
+            32 => ConnectionTrafficSecrets::Aes256Gcm { key, iv },
+            _ => unreachable!(),
         })
     }
 }