usb/dev-mtp: Fix Error double free after inotify failure

error_report_err() frees its first argument.  Freeing it again is
wrong.  Don't.

Fixes: 47287c27d0c367a89f7b2851e23a7f8b2d499dd6
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Daniel P. Berrangé <berrange@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200630090351.1247703-7-armbru@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

hw/usb/dev-mtp.c

@@ -634,7 +634,6 @@ static void usb_mtp_object_readdir(MTPState *s, MTPObject *o)
             error_reportf_err(err,
                               "usb-mtp: failed to add watch for %s: ",
                               o->path);
-            error_free(err);
         } else {
             trace_usb_mtp_file_monitor_event(s->dev.addr, o->path,
                                              "Watch Added");
@@ -1279,7 +1278,6 @@ static void usb_mtp_command(MTPState *s, MTPControl *c)
         if (err) {
             error_reportf_err(err,
                               "usb-mtp: file monitoring init failed: ");
-            error_free(err);
         } else {
             QTAILQ_INIT(&s->events);
         }