glgsg: fix shader not being applied if same across multiple buffers

See https://discourse.panda3d.org/t/multi-pass-rendering-setinitialstate-wont-apply-shader/25712

panda/src/glstuff/glGraphicsStateGuardian_src.cxx

@@ -4148,6 +4148,7 @@ end_frame(Thread *current_thread) {
     _current_shader = nullptr;
     _current_shader_context = nullptr;
   }
+  _state_shader = nullptr;
 #endif
 
   // Respecify the active texture next frame, for good measure.