mesa/util: don't ignore NULL returned from 'malloc'

We should exit from the function 'util_vasprintf'
with error code -1 for case where 'malloc'
returns NULL

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 65cfe698)