gallium/hud: check NULL return from u_upload_alloc

gallium/hud: check NULL return from u_upload_alloc

Fixes the following segmentation fault:

signal SIGSEGV: invalid address (fault address: 0x0)
 frame #0: 0x00007fffe718e117 radeonsi_dri.so hud_draw_background_quad hud_context.c:170
   167
   168 	   assert(hud->bg.num_vertices + 4 <= hud->bg.max_num_vertices);
   169
-> 170 	   vertices[num++] = (float) x1;
   171 	   vertices[num++] = (float) y1;
   172
   173 	   vertices[num++] = (float) x1;
(lldb) bt
  * frame #0: 0x00007fffe718e117 radeonsi_dri.so`hud_draw_background_quad
    frame #1: 0x00007fffe718f458 radeonsi_dri.so`hud_draw
    frame #2: 0x00007fffe712967f radeonsi_dri.so`dri_flush

Signed-off-by: Marek Olšák <marek.olsak@amd.com>