[kernel] Check for null handle arg in mailbox_call

The handle argument to `mailbox_call` is optional, so needs to be
manually checked by the syscall handler before dereferencing.

src/kernel/syscalls/mailbox.cpp

@@ -39,6 +39,8 @@ mailbox_call(
 
     data.tag = *tag;
     data.subtag = *subtag;
+
+    if (handle)
         data.handle = *handle;
 
     j6_status_t s = self->call();
@@ -47,8 +49,11 @@ mailbox_call(
 
     *tag = data.tag;
     *subtag = data.subtag;
+
+    if (handle) {
         *handle = data.handle;
         process::current().add_handle(*handle);
+    }
 
     return j6_status_ok;
 }