No memory allocation for each C_DecryptUpdate.

Before this change the allocated memory was growing after each
C_DecryptUpdate.

Author: Lars Silvén

src/lib/crypto/SymmetricAlgorithm.cpp

@@ -121,7 +121,9 @@ bool SymmetricAlgorithm::decryptUpdate(const ByteString& encryptedData, ByteStri
 	}
 
 	currentBufferSize += encryptedData.size();
-	currentAEADBuffer += encryptedData;
+	if (currentCipherMode == SymMode::GCM) {
+		currentAEADBuffer += encryptedData;
+	}
 
 	return true;
 }