allocator: token_alloc tweaks

Document what we expect callers to do, and make our examples comply, but
also explicitly initialize the unsealed pointer for defense in depth.

examples/05.sealing/identifier.cc

diff --git a/examples/05.sealing/identifier.cc b/examples/05.sealing/identifier.cc
index c66db96..08b2c5d 100644
--- a/examples/05.sealing/identifier.cc
+++ b/examples/05.sealing/identifier.cc
@@ -36,7 +36,7 @@
 	// capabilities.
 	auto [unsealed, sealed] =
 	  blocking_forever<token_allocate<Identifier>>(MALLOC_CAPABILITY, key());
-	if (sealed == nullptr)
+	if (!sealed.is_valid())
 	{
 		return nullptr;
 	}

sdk/include/token.h

diff --git a/sdk/include/token.h b/sdk/include/token.h
index 76aec99..b1e2a09 100644
--- a/sdk/include/token.h
+++ b/sdk/include/token.h
@@ -190,14 +190,20 @@
 /**
  * Type-safe helper to allocate a sealed `T*`.  Returns the sealed and unsealed
  * pointers.
+ *
+ * Callers should check the sealed capability's tag to determine success.
  */
 template<typename T>
 __always_inline std::pair<T *, Sealed<T>>
 token_allocate(Timeout *timeout, struct SObjStruct *heapCapability, SKey key)
 {
-	void *unsealed;
-	SObj  sealed = token_sealed_unsealed_alloc(
-	   timeout, heapCapability, key, sizeof(T), &unsealed);
+	/*
+	 * Explicitly initialize unsealed, since callers like to check it, and not
+	 * the sealed result, for validity.
+	 */
+	void *unsealed = nullptr;
+	SObj  sealed   = token_sealed_unsealed_alloc(
+	     timeout, heapCapability, key, sizeof(T), &unsealed);
 	return {static_cast<T *>(unsealed), Sealed<T>{sealed}};
 }
 

sdk/lib/queue/queue_compartment.cc

diff --git a/sdk/lib/queue/queue_compartment.cc b/sdk/lib/queue/queue_compartment.cc
index 9aec00a..4ed5f52 100644
--- a/sdk/lib/queue/queue_compartment.cc
+++ b/sdk/lib/queue/queue_compartment.cc
@@ -186,7 +186,7 @@
 	}
 	auto [unsealed, sealed] = token_allocate<RestrictedEndpoint>(
 	  timeout, heapCapability, receive_key());
-	if (!unsealed)
+	if (!sealed.is_valid())
 	{
 		return -ENOMEM;
 	}
@@ -208,7 +208,7 @@
 	}
 	auto [unsealed, sealed] =
 	  token_allocate<RestrictedEndpoint>(timeout, heapCapability, send_key());
-	if (!unsealed)
+	if (!sealed.is_valid())
 	{
 		return -ENOMEM;
 	}