Use dummy log callback for invalid_get_byoffset_test()
Use dummy log callback for invalid_get_byoffset_test()

- Since the test triggers a lot of intentional errors, we disable
error messages by assigning dummy callback.

Change-Id: I51b2442555cf0847a4bd9fc5073d137c7d8e6bfa
Reviewed-on: http://review.couchbase.org/62765
Reviewed-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>
Tested-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>

couchbase/forestdb,hisundar/forestdb,hisundar/forestdb,wurikiji/forestdb,couchbase/forestdb,wurikiji/forestdb