UI: Check that CEF loaded before loading integrations

Fixes a case where a certain plugin that entirely replaces OBS's own
browser plugin would cause OBS to crash with the new service
integration.  ..Yea.

UI/auth-mixer.cpp

@@ -181,6 +181,9 @@ static inline std::string get_config_str(
 
 bool MixerAuth::LoadInternal()
 {
+	if (!cef)
+		return false;
+
 	OBSBasic *main = OBSBasic::Get();
 	name = get_config_str(main, service(), "Name");
 	id = get_config_str(main, service(), "Id");

UI/auth-twitch.cpp

@@ -145,6 +145,9 @@ static inline std::string get_config_str(
 
 bool TwitchAuth::LoadInternal()
 {
+	if (!cef)
+		return false;
+
 	OBSBasic *main = OBSBasic::Get();
 	name = get_config_str(main, service(), "Name");
 	firstLoad = false;