Fixed OpenGL error when calling Shutdown without calling NewFrame

examples/sdl_opengl3_example/imgui_impl_sdl_gl3.cpp

@@ -279,15 +279,15 @@ void    ImGui_ImplSdlGL3_InvalidateDeviceObjects()
     if (g_ElementsHandle) glDeleteBuffers(1, &g_ElementsHandle);
     g_VaoHandle = g_VboHandle = g_ElementsHandle = 0;
 
-    glDetachShader(g_ShaderHandle, g_VertHandle);
-    glDeleteShader(g_VertHandle);
+    if(g_ShaderHandle && g_VertHandle ) glDetachShader(g_ShaderHandle, g_VertHandle);
+    if(g_VertHandle ) glDeleteShader(g_VertHandle);
     g_VertHandle = 0;
 
-    glDetachShader(g_ShaderHandle, g_FragHandle);
-    glDeleteShader(g_FragHandle);
+    if(g_ShaderHandle && g_FragHandle ) glDetachShader(g_ShaderHandle, g_FragHandle);
+    if(g_FragHandle ) glDeleteShader(g_FragHandle);
     g_FragHandle = 0;
 
-    glDeleteProgram(g_ShaderHandle);
+    if(g_ShaderHandle ) glDeleteProgram(g_ShaderHandle);
     g_ShaderHandle = 0;
 
     if (g_FontTexture)