Fix the crash issue for NPPM_GETNATIVELANGFILENAME

Fix notepad-plus-plus#15627

PowerEditor/src/MISC/PluginsManager/Notepad_plus_msgs.h

@@ -985,6 +985,7 @@ enum Platform { PF_UNKNOWN, PF_X86, PF_X64, PF_IA64, PF_ARM64 };
 	// Get the Current native language file name string. Use it after getting NPPN_READY notification to find out which native language is used.
 	// Users should call it with nativeLangFileName as NULL to get the required number of char (not including the terminating nul character),
 	// allocate language file name string buffer with the return value + 1, then call it again to get the current native language file name string.
+	// If there's no localization file aplied, the returned value is 0.
 	// wParam[in]: strLen is "language file name string" buffer length
 	// lParam[out]: language file name string receives all copied native language file name string
 	// Return the number of char copied/to copy

PowerEditor/src/NppBigSwitch.cpp

@@ -3761,7 +3761,11 @@ LRESULT Notepad_plus::process(HWND hwnd, UINT message, WPARAM wParam, LPARAM lPa
 
 		case NPPM_GETNATIVELANGFILENAME:
 		{
-			string fileName = _nativeLangSpeaker.getFileName();
+			const char* fn = _nativeLangSpeaker.getFileName();
+
+			if (!fn) return 0;
+
+			string fileName = fn;
 			if (lParam != 0)
 			{
 				if (fileName.length() >= static_cast<size_t>(wParam))