RFR: 8271869: AArch64: build errors with GCC11 in frame::saved_oop_result

[Alan Hayward]

GCC11 warns that r0 is null. It does not complain if r1 is used instead.

AIUI, it is undefined behaviour in C++ to call member functions without
a valid this pointer. So, technically GCC is correct to warn.

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Commit messages:
 - 8271869: AArch64: build errors with GCC11 in frame::saved_oop_result

Changes: https://git.openjdk.java.net/jdk/pull/5020/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5020&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8271869
  Stats: 6 lines in 1 file changed: 6 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5020.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5020/head:pull/5020

PR: https://git.openjdk.java.net/jdk/pull/5020