i965: Check result of make_surface() for intel_miptree_create_for_bo

Since make_surface() can fail, if the format isn't support by hw or
simlar error, we need to check the result before dereferencing it.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/mesa/drivers/dri/i965/intel_mipmap_tree.c

@@ -792,6 +792,8 @@ intel_miptree_create_for_bo(struct brw_context *brw,
                         0, 0, width, height, depth, 1, ISL_TILING_Y0_BIT,
                         ISL_SURF_USAGE_DEPTH_BIT | ISL_SURF_USAGE_TEXTURE_BIT,
                         BO_ALLOC_FOR_RENDER, pitch, bo);
+      if (!mt)
+         return NULL;
 
       brw_bo_reference(bo);