[clang-tidy] Fix bugprone-unchecked-optional-access in FunctionSizeCheck

Author: zeyi2

clang-tools-extra/clang-tidy/readability/FunctionSizeCheck.cpp

@@ -225,10 +225,12 @@ void FunctionSizeCheck::check(const MatchFinder::MatchResult &Result) {
         << ActualNumberParameters << ParameterThreshold.value();
   }
 
-  for (const auto &CSPos : FI.NestingThresholders) {
-    diag(CSPos, "nesting level %0 starts here (threshold %1)",
-         DiagnosticIDs::Note)
-        << NestingThreshold.value() + 1 << NestingThreshold.value();
+  if (NestingThreshold) {
+    for (const auto &CSPos : FI.NestingThresholders) {
+      diag(CSPos, "nesting level %0 starts here (threshold %1)",
+           DiagnosticIDs::Note)
+          << NestingThreshold.value() + 1 << NestingThreshold.value();
+    }
   }
 
   if (VariableThreshold && FI.Variables > VariableThreshold) {