r600g: check NULL return from u_upload_alloc

r600g: check NULL return from u_upload_alloc

Like done in si_state_draw.c::si_draw_vbo

u_upload_alloc can fail, i.e. set output param *ptr to NULL, for 2 reasons:
alloc fails or map fails. For both there is already a fprintf/stderr in
radeon_create_bo and radeon_bo_do_map.

In src/gallium/drivers/ it is a common usage to just avoid to crash by doing
a silent check. But defer fprintf where the error comes from, libdrm calls.

Signed-off-by: Julien Isorce <jisorce@oblong.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>