core: [Fix] NPE in GlusterSyncJob

Add validation for corrupt brick(s).

Change-Id: Ia11d8bf25a4638c6c205508b26868bfcd2f873cb
Signed-off-by: Gustavo Pedrosa <gustavo.pedrosa@eldorado.org.br>
Reviewed-on: http://gerrit.ovirt.org/23817
Reviewed-on: http://gerrit.ovirt.org/24110