u_vbuf: Fix leak

Reported by Coverity: data is heap-allocated, but only freed in the
info->index_size != 0 branch.

Signed-off-by: Ernestas Kulik <ernestas.kulik@gmail.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>
Cc: 18.2 <mesa-stable@lists.freedesktop.org>
(cherry picked from commit ea1e50cc)