Fix BIO_printf on Windows.

vsnprintf returns -1 on Windows on truncation, not the needed
size.

Change-Id: I0a9f32504127b2fb740244c3b59132e201d14234
Reviewed-on: https://boringssl-review.googlesource.com/1870
Reviewed-by: Adam Langley <agl@google.com>
1 file changed