[clang] [clang-tools-extra] [clang][dataflow] Add null-check after dereference checker (PR #84166)

[via cfe-commits]

https://github.com/whisperity commented:

(@Discookie and I had an extensive prior review at Discookie/llvm-project#1 in case someone is interested in the details.)

https://github.com/llvm/llvm-project/pull/84166