Allow to pass callbacks in varargs

[vincentisambart]

As I explained in #732 (comment), if you passed callbacks in varargs, a NULL pointer would be dereferenced and your program would crash.
I modified variadic_invoke to pass proper callbackParameters and callbackCount to rbffi_SetupCallParams, and that seems to fix the problem.

[vincentisambart]

@larskanis Hi! Would you have anything against allowing to pass callbacks via varargs arguments?

[vincentisambart]

By the way, passing callbacks in varargs might look like a rare use case, but functions of very used libraries like libcurl or sqlite use it.

For example, libcurl's curl_easy_getinfo function is declared as follows:

CURL_EXTERN CURLcode curl_easy_getinfo(CURL *curl, CURLINFO info, ...);

A library called ethon was declaring its arguments as [:pointer, :info, :pointer], even though the correct definition should be [:pointer, :info, :varargs].
That was working properly on many platforms, as using varargs or not had a similar ABI, but on ARM Macs, the ABI is different so that does not work properly. A bit more details in typhoeus/ethon#180.

The correct arguments definition needs this PR's fix to work properly.

[larskanis]

@vincentisambart Thank you for implementing this!

I merged your PR and did some fine tuning on the master branch. Let me know, if I misunderstood something! I'll try to make a new release soon, but I have only little spare time currently.

[vincentisambart]

Thanks a lot!
Sorry for the fix you had to do. It's been too long since last time I wrote C code and completely forgot about freeing memory 😅
I tried the latest master branch with my use case and it still seems to work well 😄

No worries, I totally understand spare time is a limited resource.
Looking forward to the release 😁

[jdarnok]

Thanks @vincentisambart and @larskanis for the great job you are doing here! one question regarding a release - can I know how long we have to wait for the release? there is quite a lot of people being blocked by it, including me 😄