[clang] [analyzer][NFC] Fix a warning in RegionStore.cpp (PR #157630)

[Balazs Benics via cfe-commits]

steakhal wrote:

> @steakhal Why did you discard this patch? It looks good to me at first glance.

Ah, accidentally :D I thought I already merged it. And in the `branches` section of your fork it doesn't say anything if you try to delete a branch that has an open PR. nvm.

Reopening it. @NagyDonat have look :)

https://github.com/llvm/llvm-project/pull/157630