src: use Isolate::TryGetCurrent where appropriate

In two places, we call `Isolate::GetCurrent()` even though that is
technically invalid usage of the function.
Now that V8 exposes `Isolate::TryGetCurrent()`, we can do this
in a proper way.

PR-URL: #39954
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

src/node.cc

@@ -147,7 +147,7 @@ unsigned int reverted_cve = 0;
 
 // util.h
 // Tells whether the per-process V8::Initialize() is called and
-// if it is safe to call v8::Isolate::GetCurrent().
+// if it is safe to call v8::Isolate::TryGetCurrent().
 bool v8_initialized = false;
 
 // node_internals.h

src/node_errors.cc

@@ -430,7 +430,7 @@ void OnFatalError(const char* location, const char* message) {
     FPrintF(stderr, "FATAL ERROR: %s\n", message);
   }
 
-  Isolate* isolate = Isolate::GetCurrent();
+  Isolate* isolate = Isolate::TryGetCurrent();
   Environment* env = nullptr;
   if (isolate != nullptr) {
     env = Environment::GetCurrent(isolate);

src/util.cc

@@ -128,7 +128,7 @@ BufferValue::BufferValue(Isolate* isolate, Local<Value> value) {
 
 void LowMemoryNotification() {
   if (per_process::v8_initialized) {
-    auto isolate = Isolate::GetCurrent();
+    auto isolate = Isolate::TryGetCurrent();
     if (isolate != nullptr) {
       isolate->LowMemoryNotification();
     }

src/util.h

@@ -91,7 +91,7 @@ inline T MultiplyWithOverflowCheck(T a, T b);
 
 namespace per_process {
 // Tells whether the per-process V8::Initialize() is called and
-// if it is safe to call v8::Isolate::GetCurrent().
+// if it is safe to call v8::Isolate::TryGetCurrent().
 extern bool v8_initialized;
 }  // namespace per_process