Using two calls to vsnprintf allows us to correctly determine
maximum buffer size as well as fixing garbage characters from
a missing NULL terminator.

Bug report from roland@lagoa.com.

BUG=angle:561

Change-Id: I4eb156342d9106d40783a3cf57c0c1d1889390e2
Reviewed-on: https://chromium-review.googlesource.com/186988Reviewed-by: Nicolas Capens <nicolascapens@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>