[clang] [Format] Do not crash on non-null terminated strings (PR #131299)

[via cfe-commits]

cor3ntin wrote:

It would be useful to have a repro or a stack trace here.
In particular, in `SourceManagerForFile`, `RequiresNullTerminator` is `false`, so the assert should _not_ fire on a non-null terminated file


https://github.com/llvm/llvm-project/pull/131299