swrast: always call _swrast_choose_texture_sample_func()

_swrast_choose_texture_sample_func() handles null texture object pointers
and will return the "null" sampler function which returns (0,0,0,1).  This
fixes a minor regression from ce82914f5a

src/mesa/swrast/s_context.c

@@ -476,8 +476,8 @@ _swrast_update_texture_samplers(struct gl_context *ctx)
        */
       if (tObj) {
          _mesa_update_fetch_functions(tObj);
-         swrast->TextureSample[u] = _swrast_choose_texture_sample_func(ctx, tObj);
       }
+      swrast->TextureSample[u] = _swrast_choose_texture_sample_func(ctx, tObj);
    }
 }