[clang] [clang] Fix a possible out-of-bounds read (PR #80023)

[via cfe-commits]

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@@ -1251,7 +1251,7 @@ highlightLines(StringRef FileData, unsigned StartLineNumber,
     unsigned LineLength = 0;
     for (unsigned I = 0; I <= Spelling.size(); ++I) {
       // This line is done.
-      if (isVerticalWhitespace(Spelling[I]) || I == Spelling.size()) {
+      if (I == Spelling.size() || isVerticalWhitespace(Spelling.at(I))) {
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cor3ntin wrote:

The bug was the order of the instruction,  the `at` is not doing anything useful here here. checking the size first should be necessary and sufficient.

The interesting question is why that wasn't caught by tools?
We have a repro? ( I don't insist on having one to land that though)


https://github.com/llvm/llvm-project/pull/80023