BReference: add IsSet method

Change-Id: Ibc34dbde3b81faecdc7fe66a2c07fc33d7871ce2
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3487
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>

headers/os/support/Referenceable.h

@@ -91,6 +91,11 @@ public:
 		}
 	}
 
+	bool IsSet() const
+	{
+		return fObject != NULL;
+	}
+
 	Type* Get() const
 	{
 		return fObject;
@@ -197,6 +202,11 @@ public:
 		fReference.Unset();
 	}
 
+	bool IsSet() const
+	{
+		return fReference.IsSet();
+	}
+
 	const Type* Get() const
 	{
 		return fReference.Get();