NativeScriptLanguage::unregister_binding_functions fails.

[fsecilia]

Godot version:
3.1-stable, but probably since 0b2afa2.

OS/device including version:
Arch Linux

Issue description:
NativeScriptLanguage::unregister_binding_functions() fails when binding_data.size() gets out of sync with binding_functions.size().

This happens when a gdnative library creates instance bindings for an instance of a global type, then more gdnative libraries are loaded but do not refer to that instance. Each loaded library increases the size of binding_functions, but an instance's binding_data is only updated when looking up that instance's instance bindings.

Steps to reproduce:
Consider this case. A gdnative library is loaded. It registers instance binding functions. binding_functions.size() is now 1. That library then looks up instance bindings for ResourceLoader. This creates instance bindings on ResourceLoader for that library's language. ResourceLoader's binding_data.size() is now 1.

Later, another library is loaded. This library also registers instance binding functions, so binding_functions.size() is now 2. Nothing refers to ResourceLoader's instance bindings, so its binding_data.size() is still 1.

When NativeScriptLanguage::unregister_binding_functions is called, binding_functions.size() is still 2, but ResourceLoader's binding_data.size() is still 1, causing binding_data[p_idx] to fail.

Minimal reproduction project:
This isn't trivial to reproduce, but I have a patch ready.