lib, zebra: handle failure in get chunk

lib, zebra: handle failure in get chunk

when requesting a specific label chunk (e.g. for the SRGB),
it might happen that we cannot get what we want. In this
event, we must be prepared to receive a response with no
label chunk. Without this fix, if the remote label manager
was not able to alloate the chunk we requested, we would
hang indefinitely trying to read data from the stream which
was not there.

Signed-off-by: Emanuele Di Pascale <emanuele@voltanet.io>