[clang-tidy] Fix bugprone-bad-signal-to-kill-thread crash when SIGTERM was undefined after definition.

`PP->getMacroInfo()` returns nullptr for undefined macro, which leads to null-dereference at `MI->tockens().back()`.
Stack dump:
```
 #0 0x000000000217d15a llvm::sys::PrintStackTrace(llvm::raw_ostream&) (/llvm-project/build/bin/clang-tidy+0x217d15a)
 #1 0x000000000217b17c llvm::sys::RunSignalHandlers() (/llvm-project/build/bin/clang-tidy+0x217b17c)
 #2 0x000000000217b2e3 SignalHandler(int) (/llvm-project/build/bin/clang-tidy+0x217b2e3)
 #3 0x00007f39be5b1390 __restore_rt (/lib/x86_64-linux-gnu/libpthread.so.0+0x11390)
 #4 0x0000000000593532 clang::tidy::bugprone::BadSignalToKillThreadCheck::check(clang::ast_matchers::MatchFinder::MatchResult const&) (/llvm-project/build/bin/clang-tidy+0x593532)
```

Reviewed By: hokein

Differential Revision: https://reviews.llvm.org/D85401

Author: ArcsinX

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

@@ -30,7 +30,8 @@ static Preprocessor *PP;
 
 void BadSignalToKillThreadCheck::check(const MatchFinder::MatchResult &Result) {
   const auto IsSigterm = [](const auto &KeyValue) -> bool {
-    return KeyValue.first->getName() == "SIGTERM";
+    return KeyValue.first->getName() == "SIGTERM" &&
+           KeyValue.first->hasMacroDefinition();
   };
   const auto TryExpandAsInteger =
       [](Preprocessor::macro_iterator It) -> Optional<unsigned> {

clang-tools-extra/test/clang-tidy/checkers/bugprone-bad-signal-to-kill-thread-undef-sigterm.cpp

@@ -0,0 +1,11 @@
+// RUN: clang-tidy %s --checks=-*,bugprone-bad-signal-to-kill-thread -- | count 0
+
+#define SIGTERM 15
+#undef SIGTERM // no-crash
+using pthread_t = int;
+int pthread_kill(pthread_t thread, int sig);
+
+int func() {
+  pthread_t thread;
+  return pthread_kill(thread, 0);
+}