drm/ttm: add a check against null pointer dereference

drm/ttm: add a check against null pointer dereference

When calling ttm_range_man_fini(), 'man' may be uninitialized, which may
cause a null pointer dereference bug.

Fix this by checking if it is a null pointer.

This log reveals it:

[    7.902580 ] BUG: kernel NULL pointer dereference, address: 0000000000000058
[    7.905721 ] RIP: 0010:ttm_range_man_fini+0x40/0x160
[    7.911826 ] Call Trace:
[    7.911826 ]  radeon_ttm_fini+0x167/0x210
[    7.911826 ]  radeon_bo_fini+0x15/0x40
[    7.913767 ]  rs400_fini+0x55/0x80
[    7.914358 ]  radeon_device_fini+0x3c/0x140
[    7.914358 ]  radeon_driver_unload_kms+0x5c/0xe0
[    7.914358 ]  radeon_driver_load_kms+0x13a/0x200
[    7.914358 ]  ? radeon_driver_unload_kms+0xe0/0xe0
[    7.914358 ]  drm_dev_register+0x1db/0x290
[    7.914358 ]  radeon_pci_probe+0x16a/0x230
[    7.914358 ]  local_pci_probe+0x4a/0xb0

Signed-off-by: Zheyu Ma <zheyuma97@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1626274459-8148-1-git-send-email-zheyuma97@gmail.com
Signed-off-by: Christian König <christian.koenig@amd.com>