SymmetricCipher: Add keySize(), don't rely on state for sizes

This additionally makes keySize() and blockSize() work before
setting the key and IV. Required for SSH agent decryption.

src/crypto/SymmetricCipherGcrypt.h

@@ -39,6 +39,7 @@ public:
     Q_REQUIRED_RESULT bool processInPlace(QByteArray& data, quint64 rounds);
 
     bool reset();
+    int keySize() const;
     int blockSize() const;
 
     QString errorString() const;
@@ -54,7 +55,6 @@ private:
     const SymmetricCipher::Direction m_direction;
     QByteArray m_key;
     QByteArray m_iv;
-    int m_blockSize;
     QString m_errorString;
 };