Prevent crash constructing string from nulltpr

If XDG_CURRENT_DESKTOP isn't set, getenv returns a `nullptr`.
Constructing a `std::string` from `nullptr` will throw an exception.
This prevents a crash when not using a DE, but zenity or kdialog is
installed regardless

(also fixes a typo where the fallthrough attribute wasn't applied to
anything due to a missing semicolon)

FCHost/fchost/fchost_handler_linux.cc

@@ -70,7 +70,7 @@ namespace {
 
 		switch (mode) {
 			case CefDialogHandler::FileDialogMode::FILE_DIALOG_OPEN_MULTIPLE:
-				cmdLine << " --multiple"; [[fallthrough]]
+				cmdLine << " --multiple"; [[fallthrough]];
 			case CefDialogHandler::FileDialogMode::FILE_DIALOG_OPEN:
 				cmdLine << " --getopenfilename";
 				break;
@@ -171,7 +171,8 @@ namespace {
 		// we will try to launch kdialog or zenity
 		std::string dialogExecutable;
 		// try to determine which environment we run inside
-		std::string desktop = getenv("XDG_CURRENT_DESKTOP");
+		const char* desktop_env = getenv("XDG_CURRENT_DESKTOP");
+		std::string desktop = desktop_env == nullptr ? "" : desktop_env;
 
 		const auto checkExeExists = [](const char* name) {
 			int ec = ::system((std::string(name) + " --help > /dev/null").c_str());