src: reset main_thread_id_ when it is removed

When the Environment instance associated with the main thread is
removed, reset the main_thread_id_. This will make sure calls to
GetMainEnvInst() don't return something invalid.

PR-URL: #24
Reviewed-by: Santiago Gimeno <santiago.gimeno@gmail.com>

src/nsolid/nsolid_api.cc

@@ -936,6 +936,10 @@ void EnvList::RemoveEnv(Environment* env) {
     CHECK(it != env_map_.end());
     CHECK_EQ(envinst_sp, it->second);
 #endif
+    // Remove the main thread id if the main thread is being removed.
+    if (main_thread_id_ == env->thread_id()) {
+      main_thread_id_ = 0xFFFFFFFFFFFFFFFF;
+    }
     env_map_.erase(env->thread_id());
   }