LibWeb: Don't call an absent error callback in load_sync()

Make ResourceLoader::load_sync() match load() in checking if the
error_callback is null before actually calling it.

Fixes #1623.

Libraries/LibWeb/ResourceLoader.cpp

@@ -57,7 +57,8 @@ void ResourceLoader::load_sync(const URL& url, Function<void(const ByteBuffer&)>
             loop.quit(0);
         },
         [&](auto& string) {
-            error_callback(string);
+            if (error_callback)
+                error_callback(string);
             loop.quit(0);
         });