[clang-tidy] Fix crash in readability-redundant-string-cstr

Summary: Addresses [[ https://bugs.llvm.org/show_bug.cgi?id=45286 | clang-tidy-11: Crash in DynTypedMatcher::matches during readability-redundant-string-cstr check ]]

Reviewers: aaron.ballman, alexfh, gribozavr2

Reviewed By: gribozavr2

Subscribers: xazax.hun, cfe-commits

Tags: #clang, #clang-tools-extra

Differential Revision: https://reviews.llvm.org/D76761