RFR: 8355490: Make VM_RedefineClasses::merge_constant_pools only take reference arguments

[Johan Sjölen]

Hi,

I've changed two pointer arguments to be references instead, as we bail if they are null and `assert(false)` on top of that. There are no other calls to this function than the one I fixed.

Testing: GHA Tier1

-------------

Commit messages:
 - Oops
 - Also fix the header declaration!
 - Can't be null so make it a reference

Changes: https://git.openjdk.org/jdk/pull/24848/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24848&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8355490
  Stats: 39 lines in 2 files changed: 0 ins; 8 del; 31 mod
  Patch: https://git.openjdk.org/jdk/pull/24848.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24848/head:pull/24848

PR: https://git.openjdk.org/jdk/pull/24848