monitor: Plug memory leak on QMP error

Leak introduced in commit 8a4f501..710aec9, v2.4.0.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1446117309-15322-1-git-send-email-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>
