Potential NULL pointer dereference in radeon_ttm_tt_populate

[Shaobo He]

Hello everyone,

My name is Shaobo He and I am a graduate student at University of Utah. I am 
using a static analysis tool to search for null pointer dereferences and came 
across a potentially invalid memory access in the file 
drivers/gpu/drm/radeon/radeon_ttm.c: in function `radeon_ttm_tt_populate`, 
function `radeon_ttm_tt_to_gtt` can return a NULL pointer which is dereferenced 
by the call to `drm_prime_sg_to_page_addr_arrays`.

Please let me know if it makes sense. I am looking forward to your reply.

Best,
Shaobo