Fixing memory leaks in the encryption module

Fixing memory leaks in the encryption module

Due to usage of early-returns, combined with malloc/free,
several buffers that get allocated are leaked when an error
occurs.

Several functions had potential leaks:

 - `encryptStringSymmetric` leaked `ctext`
 - `EncryptionHelper::fileDecryption` leaked `out`
 - `EncryptionHelper::fileEncryption` leaked `out`

Most of the functions had leaks of the cypher context.

This patch uses `QByteArray` as the handler for the dynamically
allocated buffers for openssl to operate on. This also removes
the need for conversions from malloc'd buffers to `QByteArray`
variables previously present in the code.

It also introduces a `CypherCtx` thin wrapper class to provide
a leak-free handling of `EVP_CIPHER_CTX`.