java/openjdk11(-jre): fix build with clang 12

During an exp-run for llvm 12 (see bug 255570), it turned out that
java/openjdk11 and java/openjdk11-jre do not build with clang 12.0.0:

Creating interim jimage
Compiling 2 files for BUILD_DEMO_Notepad

This is due to a missing backport of an upstream commit:

commit c484d8904285652246c3af212a4211b9a8955149
Author: Thomas Stuefe <stuefe@openjdk.org>
Date: Tue Mar 16 05:49:01 2021 +0000

8263557: Possible NULL dereference in Arena::destruct_contents()

Reviewed-by: kbarrett, coleenp

Approved by: maintainer timeout (2 weeks)
PR: 255900
MFH: 2021Q2