[GDNative] fix crash at shutdown when using singleton libraries and NativeScript

When a singleton library was exposing NativeScript functionality,
the NativeScriptLanguage would attempt to terminate the library at
shutdown.

Since the GDNative module itself handles singleton libraries,
it closes all singleton libraries at shutdown as well. This double free
could cause a crash, since the library referenced would no longer be alive.

modules/gdnative/gdnative.cpp

@@ -414,6 +414,7 @@ bool GDNative::terminate() {
 	if (error || !library_terminate) {
 		OS::get_singleton()->close_dynamic_library(native_handle);
 		native_handle = NULL;
+		initialized = false;
 		return true;
 	}
 

modules/gdnative/nativescript/nativescript.cpp

@@ -1036,8 +1036,16 @@ NativeScriptLanguage::~NativeScriptLanguage() {
 
 	for (Map<String, Ref<GDNative> >::Element *L = NSL->library_gdnatives.front(); L; L = L->next()) {
 
-		if (L->get().is_valid())
-			L->get()->terminate();
+		Ref<GDNative> lib = L->get();
+		// only shut down valid libs, duh!
+		if (lib.is_valid()) {
+
+			// If it's a singleton-library then the gdnative module
+			// manages the destruction at engine shutdown, not NativeScript.
+			if (!lib->get_library()->is_singleton()) {
+				lib->terminate();
+			}
+		}
 	}
 
 	NSL->library_classes.clear();
@@ -1641,10 +1649,19 @@ void NativeReloadNode::_notification(int p_what) {
 					continue;
 				}
 
+				// Don't unload what should not be reloaded!
 				if (!gdn->get_library()->is_reloadable()) {
 					continue;
 				}
 
+				// singleton libraries might have alive pointers living inside the
+				// editor. Also reloading a singleton library would mean that
+				// the singleton entry will not be called again, as this only
+				// happens at engine startup.
+				if (gdn->get_library()->is_singleton()) {
+					continue;
+				}
+
 				gdn->terminate();
 			}
 
@@ -1672,6 +1689,12 @@ void NativeReloadNode::_notification(int p_what) {
 					continue;
 				}
 
+				// since singleton libraries are not unloaded there is no point
+				// in loading them again.
+				if (!gdn->get_library()->is_singleton()) {
+					continue;
+				}
+
 				if (!gdn->initialize()) {
 					libs_to_remove.insert(L->key());
 					continue;