[VM/Runtime] Ensure an isolate exists before calling Dart_ShutdownIsolate()

Fixes https://github.com/dart-lang/sdk/issues/52892
TEST=ci

Change-Id: I4f0aa64642d4ba8416ef353dfca7371adc7e6bba
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/313543
Reviewed-by: Ryan Macnak <rmacnak@google.com>
Commit-Queue: Siva Annamalai <asiva@google.com>

runtime/bin/error_exit.cc

@@ -21,7 +21,11 @@ void ErrorExit(int exit_code, const char* format, ...) {
   Syslog::VPrintErr(format, arguments);
   va_end(arguments);
 
-  Dart_ShutdownIsolate();
+  // Sometimes ErrorExit is called even before we have entered an isolate.
+  // We need to shutdown the isolate only if one exists.
+  if (Dart_CurrentIsolate() != nullptr) {
+    Dart_ShutdownIsolate();
+  }
 
   // Terminate process exit-code handler.
   Process::TerminateExitCodeHandler();