No need to return an empty value from object-add (it would also leak
if the command failed).  While at it, remove the "if" around object_unref
since object_unref handles NULL arguments just fine.

Reported-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20200325184723.2029630-4-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>