Segfault in setEphemeralKey

Reported in https://github.com/daurnimator/lua-http/issues/153

```lua
local openssl_ctx = require "openssl.ssl.context"
local openssl_pkey = require "openssl.pkey"

local ctx = openssl_ctx.new("TLS", false)
ctx:setEphemeralKey(openssl_pkey.new{ type = "EC", curve = "prime256v1" })
```