Fixes the flaky mutable_object_test (ray-project#44413)

Based on a local issue I resolved, I believe this commit should fix the
flaky mutable_object_test in ray-project#44396.

The header of the mutable object was not being explicitly initialized.
In some cases, this caused deadlock due to a spinlock backed by garbage memory.

This commit explicitly initializes the header, which should resolve the
issue.

Tested: mutable_object_test

Signed-off-by: Jack Humphries <[email protected]>

Author: jackhumphries

src/ray/object_manager/plasma/test/mutable_object_test.cc

@@ -113,7 +113,9 @@ std::unique_ptr<plasma::MutableObject> MakeObject() {
   info.allocated_size = kPayloadSize;
 
   uint8_t *ptr = static_cast<uint8_t *>(malloc(kSize));
-  return std::make_unique<plasma::MutableObject>(ptr, info);
+  auto ret = std::make_unique<plasma::MutableObject>(ptr, info);
+  ret->header->Init();
+  return ret;
 }
 
 }  // namespace