Fix va_list reuse

[junjihashimoto]

LOG function calls vsnprintf with the same va_list twice, but it is not allowed.
It throws segfault sometimes.
This PR copies the va_list not to reuse it.
https://stackoverflow.com/questions/2288680/reuse-of-va-list

[austinvhuang]

Thanks for the catch. it feels kind of messy to call vsnprintf twice in the first place to get the size, and then to need to copy the va_list to facilitate that, but I don't have a better solution for now so we'll go with this at least until we find a cleaner solution.