Fix SSLCertificates::ConvertDER

Fix #17. Use temporary buffer according to https://www.openssl.org/docs/crypto/d2i_X509.html#WARNINGS to avoid SEGFAULT at OPENSSL_free.
Merge branch 'fix-ConvertDer' of https://github.com/tbeu/SynFTP into fix-ConvertDer

Conflicts:
	src/SSLCertificates.cpp