drm/ttm: add missing NULL checks

My local syzbot instance hit GPF in ttm_bo_release().
Unfortunately, syzbot didn't produce a reproducer for this, but I
found out possible scenario:

drm_gem_vram_create()            <-- drm_gem_vram_object kzalloced
				     (bo embedded in this object)
  ttm_bo_init()
    ttm_bo_init_reserved()
      ttm_resource_alloc()
        man->func->alloc()       <-- allocation failure
      ttm_bo_put()
	ttm_bo_release()
	  ttm_mem_io_free()      <-- bo->resource == NULL passed
				     as second argument
	     *GPF*

Added NULL check inside ttm_mem_io_free() to prevent reported GPF and
make this function NULL save in future.

Same problem was in ttm_bo_move_to_lru_tail() as Christian reported.
ttm_bo_move_to_lru_tail() is called in ttm_bo_release() and mem pointer
can be NULL as well as in ttm_mem_io_free().

Fail log:

KASAN: null-ptr-deref in range [0x0000000000000020-0x0000000000000027]
...
RIP: 0010:ttm_mem_io_free+0x28/0x170 drivers/gpu/drm/ttm/ttm_bo_util.c:66
..
Call Trace:
 ttm_bo_release+0xd94/0x10a0 drivers/gpu/drm/ttm/ttm_bo.c:422
 kref_put include/linux/kref.h:65 [inline]
 ttm_bo_put drivers/gpu/drm/ttm/ttm_bo.c:470 [inline]
 ttm_bo_init_reserved+0x7cb/0x960 drivers/gpu/drm/ttm/ttm_bo.c:1050
 ttm_bo_init+0x105/0x270 drivers/gpu/drm/ttm/ttm_bo.c:1074
 drm_gem_vram_create+0x332/0x4c0 drivers/gpu/drm/drm_gem_vram_helper.c:228

Fixes: d3116756 ("drm/ttm: rename bo->mem and make it a pointer")
Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210708112518.17271-1-paskripkin@gmail.com