[clang] [clang][CodeGen] Fix memory size calculation in EmitNullBaseClassInitialization (PR #184558)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

Does this fix #42101 ?  If it does, can you mention that in the commit message, and add a testcase?

https://github.com/llvm/llvm-project/pull/184558