Set the fops_handle to nullptr after calling the fileops destructor
Set the fops_handle to nullptr after calling the fileops destructor

The fops_handle pointer needs to be set to nullptr to ensure to
prevent illegal acess in subsequent fileops calls.

SUMMARY: ThreadSanitizer: heap-use-after-free /home/couchbase/jenkins/workspace/ep-engine-threadsanitizer-master/ep-engine/src/forest-kvstore/forest-kvstore.cc:2068 ffs_get_errno_str(fdb_fileops_handle_opaque*, char*, unsigned long)
====================================
WARNING: ThreadSanitizer: heap-use-after-free (pid=113872)
Read of size 8 at 0x7d08000e44d0 by thread T17 (mutexes: write M3273):

Previous write of size 8 at 0x7d08000e44d0 by thread T17 (mutexes: write M3273):

Change-Id: I2571b99ef74aaf865d43426c0dd0b0ad5fd4d2c2
Reviewed-on: http://review.couchbase.org/65871
Tested-by: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Chiyoung Seo <fcffc23db27b19ee4ebba46695ee6b50e09216a2@couchbase.com>

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