ttm->sg needs to be checked before accessing its child member.

Call Trace:
 amdgpu_ttm_backend_destroy+0x12/0x70 [amdgpu]
 ttm_bo_cleanup_memtype_use+0x3a/0x60 [ttm]
 ttm_bo_release+0x17d/0x300 [ttm]
 amdgpu_bo_unref+0x1a/0x30 [amdgpu]
 amdgpu_amdkfd_gpuvm_alloc_memory_of_gpu+0x78b/0x8b0 [amdgpu]
 kfd_ioctl_alloc_memory_of_gpu+0x118/0x220 [amdgpu]
 kfd_ioctl+0x222/0x400 [amdgpu]
 ? kfd_dev_is_large_bar+0x90/0x90 [amdgpu]
 __x64_sys_ioctl+0x8e/0xd0
 ? __context_tracking_exit+0x52/0x90
 do_syscall_64+0x33/0x80
 entry_SYSCALL_64_after_hwframe+0x44/0xa9
RIP: 0033:0x7f97f264d317
Code: b3 66 90 48 8b 05 71 4b 2d 00 64 c7 00 26 00 00 00 48 c7 c0 ff ff ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 b8 10 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 41 4b 2d 00 f7 d8 64 89 01 48
RSP: 002b:00007ffdb402c338 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007f97f3cc63a0 RCX: 00007f97f264d317
RDX: 00007ffdb402c380 RSI: 00000000c0284b16 RDI: 0000000000000003
RBP: 00007ffdb402c380 R08: 00007ffdb402c428 R09: 00000000c4000004
R10: 00000000c4000004 R11: 0000000000000246 R12: 00000000c0284b16
R13: 0000000000000003 R14: 00007f97f3cc63a0 R15: 00007f8836200000
Signed-off-by: NGuchun Chen <guchun.chen@amd.com>
Acked-by: NChristian König <christian.koenig@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>