Revert "Remove unneeded va_copy"

This reverts commit 485ef21b95e257e9d9cbcaa804c3c164f1f49a80.

Apparently the va_copy IS needed, because without this code
we get segfaults in some cases.

Closes #253.

@nwellnhof, can you have a look at this issue and comment?
I understand that this code was removed for portability reasons.
Is there an alternative solution?