syncop: Invoke dict_unref() in inodelk only if dictionary is not NULL

In the absence of this check, logs can get flooded with messages like this when rebalance is run:

[2014-09-04 17:48:07.148262] W [dict.c:480:dict_unref] (-->/lib64/libc.so.6()
[0x30daa47a00] (-->/usr/local/lib/libglusterfs.so.0(synctask_wrap+0x12)
[0x7fa20b7c6ec2]
(-->/usr/local/lib/glusterfs/3.7dev/xlator/cluster/distribute.so(dht_migrate_file+0x23f)
[0x7fa200fdb58f]))) 0-dict: dict is NULL

Change-Id: I4c93e4485293b35d86ba07df4d583d2758ec3f49
BUG: 1138395
Signed-off-by: Vijay Bellur <vbellur@redhat.com>
Reviewed-on-master: http://review.gluster.org/8601
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Shyamsundar Ranganathan <srangana@redhat.com>
Reviewed-on: http://review.gluster.org/8782