fix possible segfault in assignment

src/rexbacklight.c

@@ -158,6 +158,10 @@ void fade_out(int iv, int fv, int ms_duration, int steps){
 		step_delta += step_inc;
 	}
 	fd = fopen(backlight_file, "w+");
+	if(!fd){
+		io_error_2(IO_ERROR_OPEN, IO_ERROR_FILE, backlight_dir, backlight_file);
+		return;
+	}
 	fprintf(fd, "%d", fv);
 
 	fclose(fd);