[clang-tools-extra] [NFC] Fix potential nullptr dereference. (PR #138283)

[Baranov Victor via cfe-commits]

vbvictor wrote:

Did you run code sanitizer over all clang-tidy code or you only interested in this particular file?
I think there are many more places where such `nullptr` dereference can happen, e.g.
https://github.com/llvm/llvm-project/blob/52d2b589b2d7adc3e2b4ea503df4131e088de61f/clang-tools-extra/clang-tidy/bugprone/InaccurateEraseCheck.cpp#L39-L43
https://github.com/llvm/llvm-project/blob/52d2b589b2d7adc3e2b4ea503df4131e088de61f/clang-tools-extra/clang-tidy/bugprone/IncorrectRoundingsCheck.cpp#L51-L52

https://github.com/llvm/llvm-project/pull/138283