dict: de-allocate data on dict-set failures

Found the bug while browsing code. Very difficult to hit this though.

Change-Id: I84bd5d23d19b6aa16866fc0f1b56aa22cb8a5f1f
BUG: 1079215
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/7310
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>