crypto: fix openssl_err_str

openssl_err_str is used for reporting OpenSSL errors. It calls
crypto_ERR_* functions using FFI. There's a typo in the code:
ffi.crypto_ERR_error_string is used instead of ffi.C.*.

We don't normally step on this, because OpenSSL doesn't return errors
in our configuration, but if it did for some reason (e.g. a cipher was
disabled in the library), we'd get a confusing error message.

NO_DOC=bug fix
NO_TEST=occur only on internal error
NO_CHANGELOG=occur only on internal error

src/lua/crypto.lua

@@ -75,7 +75,7 @@ ffi.cdef[[
 ]]
 
 local function openssl_err_str()
-  return ffi.string(ffi.crypto_ERR_error_string(ffi.C.crypto_ERR_get_error(), nil))
+  return ffi.string(ffi.C.crypto_ERR_error_string(ffi.C.crypto_ERR_get_error(), nil))
 end
 
 local digests = {}