cppcheck: call va_end in _internal_error

fix for:
    util/error_util.c:38: error: va_list 'va_args' was opened but not
    closed by va_end()

This makes the code more copy-pastable, if nothing else

util/error_util.c

@@ -34,6 +34,7 @@ _internal_error (const char *format, ...)
     fprintf (stderr, "Internal error: ");
     vfprintf (stderr, format, va_args);
 
+    va_end (va_args);
     exit (1);
 }