Fix padding handling in SymmetricCipherStream.

The implementation had two issues:
- It didn't add a block full of padding when the input size was a multiple
  of the block size.
- It didn't strip the padding when reading data.

tests/TestSymmetricCipher.h

@@ -29,6 +29,7 @@ private Q_SLOTS:
     void testAes256CbcEncryption();
     void testAes256CbcDecryption();
     void testSalsa20();
+    void testPadding();
 };
 
 #endif // KEEPASSX_TESTSYMMETRICCIPHER_H