radv: fix double free corruption in radv_alloc_memory()

radv: fix double free corruption in radv_alloc_memory()

If the driver fails to allocate memory for some reasons, it shouldn't
free the 'mem' object twice.

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2302
Fixes: 825ddfee599 ("radv: Handle device memory alloc failure with normal free.")
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3508>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3508>
(cherry picked from commit bd51538d285ad0ef488df60e24eaa08b9da20c28)