A single va_list may not be used twice. Nothing calls BIO_vprintf and it just
(v)snprintfs into a buffer anyway, so remove it. If it's actually needed, we
can fiddle with va_copy and the lack of it in C89 later, but anything that
actually cares can just assemble the output externally.
Add a test in bio_test.c.
BUG=399546
Change-Id: Ia40a68b31cb5984d817e9c55351f49d9d6c964c1
Reviewed-on: https://boringssl-review.googlesource.com/1391
Reviewed-by: Adam Langley <agl@google.com>
aa4efe7669