[clang-tidy] Fix a crash in bugprone-not-null-terminated-result check…

… when `__STDC_WANT_LIB_EXT1__` is not a literal.

If `__STDC_WANT_LIB_EXT1__` is not a literal (e.g. `#define __STDC_WANT_LIB_EXT1__ ((unsigned)1)`) bugprone-not-null-terminated-result check crashes.
Stack dump:
```
 #0 0x0000000002185e6a llvm::sys::PrintStackTrace(llvm::raw_ostream&) (/llvm-project/build/bin/clang-tidy+0x2185e6a)
 #1 0x0000000002183e8c llvm::sys::RunSignalHandlers() (/llvm-project/build/bin/clang-tidy+0x2183e8c)
 rust-lang#2 0x0000000002183ff3 SignalHandler(int) (/llvm-project/build/bin/clang-tidy+0x2183ff3)
 rust-lang#3 0x00007f08d91b1390 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x11390)
 rust-lang#4 0x00000000021338bb llvm::StringRef::getAsInteger(unsigned int, llvm::APInt&) const (/llvm-project/build/bin/clang-tidy+0x21338bb)
 rust-lang#5 0x000000000052051c clang::tidy::bugprone::NotNullTerminatedResultCheck::check(clang::ast_matchers::MatchFinder::MatchResult const&) (/llvm-project/build/bin/clang-tidy+0x52051c)
```

Reviewed By: hokein

Differential Revision: https://reviews.llvm.org/D85525

clang-tools-extra/clang-tidy/bugprone/NotNullTerminatedResultCheck.cpp

@@ -805,10 +805,12 @@ void NotNullTerminatedResultCheck::check(
         // PP->getMacroInfo() returns nullptr if macro has no definition.
         if (MI) {
           const auto &T = MI->tokens().back();
-          StringRef ValueStr = StringRef(T.getLiteralData(), T.getLength());
-          llvm::APInt IntValue;
-          ValueStr.getAsInteger(10, IntValue);
-          AreSafeFunctionsWanted = IntValue.getZExtValue();
+          if (T.isLiteral() && T.getLiteralData()) {
+            StringRef ValueStr = StringRef(T.getLiteralData(), T.getLength());
+            llvm::APInt IntValue;
+            ValueStr.getAsInteger(10, IntValue);
+            AreSafeFunctionsWanted = IntValue.getZExtValue();
+          }
         }
       }
 

clang-tools-extra/test/clang-tidy/checkers/bugprone-not-null-terminated-result-stdc-want-lib-ext1-not-a-literal.c

@@ -0,0 +1,16 @@
+// RUN: %check_clang_tidy %s bugprone-not-null-terminated-result %t -- \
+// RUN: -- -std=c11 -I %S/Inputs/bugprone-not-null-terminated-result
+
+#include "not-null-terminated-result-c.h"
+
+#define __STDC_LIB_EXT1__ 1
+#define __STDC_WANT_LIB_EXT1__ ((unsigned)1)
+
+void f(const char *src) {
+  char dest[13];
+  memcpy_s(dest, 13, src, strlen(src) - 1);
+  // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: the result from calling 'memcpy_s' is not null-terminated [bugprone-not-null-terminated-result]
+  // CHECK-FIXES: char dest[14];
+  // CHECK-FIXES-NEXT: strncpy_s(dest, 14, src, strlen(src) - 1);
+}
+