[riscv-port] RFR: 8279664: riscv: JFR crashes at 0x0

[Xiaolin Zheng]

Hi team,

This is a preview fix for the JFR-related crash - maybe the comment in this patch should be polished.
TIL that complex problems often result from seemingly negligible changes :-)
I need to cautiously test this change, though it seems no harm, to guarantee that it is safe and to ensure the problem is gone so please stay a little tuned.

Thanks,
Xiaolin

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Commit messages:
 - Fix sp-related undefined behavior

Changes: https://git.openjdk.java.net/riscv-port/pull/49/files
 Webrev: https://webrevs.openjdk.java.net/?repo=riscv-port&pr=49&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279664
  Stats: 6 lines in 1 file changed: 3 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/riscv-port/pull/49.diff
  Fetch: git fetch https://git.openjdk.java.net/riscv-port pull/49/head:pull/49

PR: https://git.openjdk.java.net/riscv-port/pull/49