mesa: reset linked_stages bitmask when re-linking

34953f8907 added this bitmask but it wasn't being reset when
a program was relinked. If a stage was removed from the new
program then it could case a crash as we expect the linked shader
for that stage to not be null.

Fixes crashes in:
ESEXT-CTS.tessellation_shader.single.xfb_captures_data_from_correct_stage
ES31-CTS.core.tessellation_shader.single.xfb_captures_data_from_correct_stage

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98917

src/mesa/main/shaderobj.c

@@ -348,6 +348,8 @@ _mesa_clear_shader_program_data(struct gl_context *ctx,
       }
    }
 
+   shProg->data->linked_stages = 0;
+
    if (shProg->data->UniformStorage) {
       for (unsigned i = 0; i < shProg->data->NumUniformStorage; ++i)
          _mesa_uniform_detach_all_driver_storage(&shProg->data->