Clean up EVP_PKEY_CTX_meth_dup()

Clean up EVP_PKEY_CTX_meth_dup()

Explicitly check against NULL, replace malloc() plus manual zeroing with
calloc(). Use EVP_PKEY_up_ref() rather than handrolling it and use a more
normal error idiom.

There still seems to be a bug in here in that the ENGINE's refcount isn't
bumped, but that will be investigated and fixed separately.

ok jsing