Don't try to create textures for empty names.

It's not designed to work with empty file names, because the "existFile" function in irrlicht returns true for directories, our file manager doesn't check that it's a directory and generally strange things happen.

This solves a problem with luna track that doesn't have a screenshot and possibly with some broken add-ons, so that valgrind doesn't complain anymore.

Benau, feel free to revert it if you have better solution.

src/graphics/stk_tex_manager.cpp

@@ -147,6 +147,12 @@ video::ITexture* STKTexManager::getTexture(const std::string& path,
                                            TexConfig* tc, bool no_upload,
                                            bool create_if_unfound)
 {
+    if (path.empty())
+    {
+        Log::error("STKTexManager", "Texture name is empty.");
+        return NULL;
+    }
+
     auto ret = m_all_textures.find(path);
     if (!no_upload && ret != m_all_textures.end())
         return ret->second;