shader auto reload

src/asset.cpp

@@ -220,12 +220,15 @@ glm::uint8_t* Asset::read_all()
 void Asset::close()
 {
 #ifdef __ANDROID__
-    AAsset_close(m_asset);
+    if (m_asset)
+        AAsset_close(m_asset);
+    m_asset = nullptr;
 #else
     if (m_fp)
         fclose(m_fp);
     if (m_data)
         delete m_data;
     m_data = nullptr;
+    m_fp = nullptr;
 #endif
 }