Some builds (depends on crypto engine support?) of OpenSSL

Some builds (depends on crypto engine support?) of OpenSSL
0.9.7x have EVP_DigestFinal function which which clears all of
EVP_MD_CTX.  This makes pgcrypto crash in functions which
re-use one digest context several times: hmac() and crypt()
with md5 algorithm.

Following patch fixes it by carring the digest info around
EVP_DigestFinal and re-initializing cipher.

Marko Kreen.