[clang-tidy] Fix a crash in bugprone-not-null-terminated-result check when __STDC_WANT_LIB_EXT1__ was undefined after definition.

PP->getMacroInfo() returns nullptr for undefined macro, so we need to check this return value before dereference.
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)
 #2 0x0000000002183ff3 SignalHandler(int) (/llvm-project/build/bin/clang-tidy+0x2183ff3)
 #3 0x00007f37df9b1390 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x11390)
 #4 0x000000000052054e clang::tidy::bugprone::NotNullTerminatedResultCheck::check(clang::ast_matchers::MatchFinder::MatchResult const&) (/llvm-project/build/bin/clang-tidy+0x52054e)
```

Reviewed By: hokein

Differential Revision: https://reviews.llvm.org/D85523

Author: ArcsinX

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

@@ -802,11 +802,14 @@ void NotNullTerminatedResultCheck::check(
     while (It != PP->macro_end() && !AreSafeFunctionsWanted.hasValue()) {
       if (It->first->getName() == "__STDC_WANT_LIB_EXT1__") {
         const auto *MI = PP->getMacroInfo(It->first);
-        const auto &T = MI->tokens().back();
-        StringRef ValueStr = StringRef(T.getLiteralData(), T.getLength());
-        llvm::APInt IntValue;
-        ValueStr.getAsInteger(10, IntValue);
-        AreSafeFunctionsWanted = IntValue.getZExtValue();
+        // 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();
+        }
       }
 
       ++It;

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

@@ -0,0 +1,17 @@
+// 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__ 1
+#undef __STDC_WANT_LIB_EXT1__
+
+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);
+}
+