[-Wunsafe-buffer-usage] Fix a potential overflow bug reported by llvm#126334 (llvm#129169)

`MeasureTokenLength` may return an unsigned 0 representing failure in
obtaining length of a token. The analysis now gives up on such cases.
Otherwise, there might be issues caused by unsigned integer "overflow".

Author: ziqingluo-90

clang/lib/Analysis/UnsafeBufferUsage.cpp

@@ -2364,12 +2364,13 @@ template <typename NodeTy>
 static std::optional<SourceLocation>
 getEndCharLoc(const NodeTy *Node, const SourceManager &SM,
               const LangOptions &LangOpts) {
-  unsigned TkLen = Lexer::MeasureTokenLength(Node->getEndLoc(), SM, LangOpts);
-  SourceLocation Loc = Node->getEndLoc().getLocWithOffset(TkLen - 1);
-
-  if (Loc.isValid())
-    return Loc;
+  if (unsigned TkLen =
+          Lexer::MeasureTokenLength(Node->getEndLoc(), SM, LangOpts)) {
+    SourceLocation Loc = Node->getEndLoc().getLocWithOffset(TkLen - 1);
 
+    if (Loc.isValid())
+      return Loc;
+  }
   return std::nullopt;
 }