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>

src/gallium/drivers/r600/r600_state_common.c

@@ -1746,6 +1746,10 @@ static void r600_draw_vbo(struct pipe_context *ctx, const struct pipe_draw_info
 
 			u_upload_alloc(ctx->stream_uploader, start, count * 2,
                                        256, &out_offset, &out_buffer, &ptr);
+			if (unlikely(!ptr)) {
+				pipe_resource_reference(&ib.buffer, NULL);
+				return;
+			}
 
 			util_shorten_ubyte_elts_to_userptr(
 						&rctx->b.b, &ib, 0, 0, ib.offset + start, count, ptr);