libglusterfs/dict: fix infinite loop in dict_keys_join()

         - missing "pairs = next" caused infinite loop

Change-Id: I3edc4f50473f7498815c73e1066167392718fddf
BUG: 905871
Signed-off-by: Vijaykumar Koppad <vkoppad@redhat.com>
Reviewed-on: http://review.gluster.org/4728
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>