Make mailbox clearing idempotent.

Change-Id: Ib07df13119eee7b4d916688bdb68a0760aedccaa
diff --git a/test/vmapi/primary_with_secondaries/interrupts.c b/test/vmapi/primary_with_secondaries/interrupts.c
index c18c8c9..ad61833 100644
--- a/test/vmapi/primary_with_secondaries/interrupts.c
+++ b/test/vmapi/primary_with_secondaries/interrupts.c
@@ -181,7 +181,6 @@
 	hf_interrupt_inject(SERVICE_VM0, 0, EXTERNAL_INTERRUPT_ID_C);
 	run_res = hf_vcpu_run(SERVICE_VM0, 0);
 	EXPECT_EQ(run_res.code, HF_VCPU_RUN_WAIT_FOR_INTERRUPT);
-	EXPECT_EQ(hf_mailbox_clear(), -1);
 
 	/*
 	 * Now send a message to the secondary to enable the interrupt ID, and
diff --git a/test/vmapi/primary_with_secondaries/mailbox.c b/test/vmapi/primary_with_secondaries/mailbox.c
index c54cdb4..bd5b588 100644
--- a/test/vmapi/primary_with_secondaries/mailbox.c
+++ b/test/vmapi/primary_with_secondaries/mailbox.c
@@ -61,6 +61,16 @@
 }
 
 /**
+ * Clearing an empty mailbox is a noop.
+ */
+TEST(mailbox, clear_empty)
+{
+	EXPECT_EQ(hf_mailbox_clear(), 0);
+	EXPECT_EQ(hf_mailbox_clear(), 0);
+	EXPECT_EQ(hf_mailbox_clear(), 0);
+}
+
+/**
  * Send and receive the same message from the echo VM.
  */
 TEST(mailbox, echo)