drm/ttm: check null pointer before accessing when swapping

In the Linux kernel, the following vulnerability has been resolved:

drm/ttm: check null pointer before accessing when swapping

Add a check to avoid null pointer dereference as below:

[ 90.002283] general protection fault, probably for non-canonical
address 0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN NOPTI
[ 90.002292] KASAN: null-ptr-deref in range
[0x0000000000000000-0x0000000000000007]
[ 90.002346] ? exc_general_protection+0x159/0x240
[ 90.002352] ? asm_exc_general_protection+0x26/0x30
[ 90.002357] ? ttm_bo_evict_swapout_allowable+0x322/0x5e0 [ttm]
[ 90.002365] ? ttm_bo_evict_swapout_allowable+0x42e/0x5e0 [ttm]
[ 90.002373] ttm_bo_swapout+0x134/0x7f0 [ttm]
[ 90.002383] ? __pfx_ttm_bo_swapout+0x10/0x10 [ttm]
[ 90.002391] ? lock_acquire+0x44d/0x4f0
[ 90.002398] ? ttm_device_swapout+0xa5/0x260 [ttm]
[ 90.002412] ? lock_acquired+0x355/0xa00
[ 90.002416] ? do_raw_spin_trylock+0xb6/0x190
[ 90.002421] ? __pfx_lock_acquired+0x10/0x10
[ 90.002426] ? ttm_global_swapout+0x25/0x210 [ttm]
[ 90.002442] ttm_device_swapout+0x198/0x260 [ttm]
[ 90.002456] ? __pfx_ttm_device_swapout+0x10/0x10 [ttm]
[ 90.002472] ttm_global_swapout+0x75/0x210 [ttm]
[ 90.002486] ttm_tt_populate+0x187/0x3f0 [ttm]
[ 90.002501] ttm_bo_handle_move_mem+0x437/0x590 [ttm]
[ 90.002517] ttm_bo_validate+0x275/0x430 [ttm]
[ 90.002530] ? __pfx_ttm_bo_validate+0x10/0x10 [ttm]
[ 90.002544] ? kasan_save_stack+0x33/0x60
[ 90.002550] ? kasan_set_track+0x25/0x30
[ 90.002554] ? __kasan_kmalloc+0x8f/0xa0
[ 90.002558] ? amdgpu_gtt_mgr_new+0x81/0x420 [amdgpu]
[ 90.003023] ? ttm_resource_alloc+0xf6/0x220 [ttm]
[ 90.003038] amdgpu_bo_pin_restricted+0x2dd/0x8b0 [amdgpu]
[ 90.003210] ? __x64_sys_ioctl+0x131/0x1a0
[ 90.003210] ? do_syscall_64+0x60/0x90