Found with Clang's `scan-build` tool.
The error value assigned to `ret` is not returned, meaning that the
selftest always succeeds. Ensure the error value is propagated back to
the caller.
The error code POLARSSL_ERR_X509_PASSWORD_MISMATCH is now properly
returned in case of an encryption failure in the padding. The
POLARSSL_ERR_X509_PASSWORD_REQUIRED error code is only returned for PEM
formatted private keys as for DER formatted ones it is impossible to
distinguish if a DER blob is PKCS#8 encrypted or not.
