mesa/util: don't use the same 'va_list' instance twice

The first usage of the 'va_list' instance could change it.

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Fixes: 864148d6 "util: add util_vasprintf() for Windows (v2)"
Signed-off-by: Andrii Simiklit <andrii.simiklit@globallogic.com>
(cherry picked from commit 570cacba)