um: virtio_uml: free command if adding to virtqueue failed

If adding the command fails (i.e. the virtqueue is broken) then free it
again if the function allocated a new buffer for it.

Fixes: 68f5d3f3 ("um: add PCI over virtio emulation driver")
Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>