Fix: SEGFAULT caused by va_list usage in error message generation

[liss-h]

Fixes a SEGFAULT caused by invalid va_list usage.

Bug Description

Serd's API is written in terms of vprintf compatibility (i.e. you need to generate the message via v*printf* and char const *fmt, va_list args)

Since we want to put the message in a buffer instead of printing it directly, we are using vsnprintf. This involves two calls to vsnprintf, one to figure out how big the buffer should be and another one to actually fill the buffer with the message. As it turns out the first call to vsnprintf consumed the va_list, therefore the second call to vsnprintf would overread into garbage. Sometimes this overread would just produce garbage messages, sometimes it would trigger a SEGFAULT.

Fix

We just need to va_copy the list for the first call. Additionally, I added some more stuff to make the code more robust.

[bigerl]

Thx for the PR. Changes look good. This was a sneaky one!