[compiler-rt] [MSan] Fix wrong unpoison size in SignalAction (PR #144071)

[Florian Mayer via llvm-commits]

fmayer wrote:

lgtm. as a sidenote, pressing this icon helps bring the change back to the reviewer's attention

![image](https://github.com/user-attachments/assets/6967f736-554e-4e6d-9b05-a3710fa73583)

https://github.com/llvm/llvm-project/pull/144071