cbbackup: Multiple incremental backup fail when the data is updated

Steps to reproduce: Using the latest 3.0 build-697:
1. Add 1000 items to the bucket
2. Take a full backup
3. Update all the items
4. Take an incremental backup
5. The cbbackup throws error and hangs:
output from cbbackup >>>>>>

Exception in thread w0:
Traceback (most recent call last):
File "/usr/lib/python2.7/threading.py", line 551, in __bootstrap_inner
self.run()
File "/usr/lib/python2.7/threading.py", line 504, in run
self._target(*self.args, **self._kwargs)
File "/opt/couchbase/lib/python/pump.py", line 279, in run_worker
curx)
File "/opt/couchbase/lib/python/pump_bfd2.py", line 20, in check_spec
getattr(opts, "mode", "diff"))
File "/opt/couchbase/lib/python/pump_bfd.py", line 272, in find_seqno
for logpair in flogs:
TypeError: 'NoneType' object is not iterable

Exception in thread w1:
Traceback (most recent call last):
File "/usr/lib/python2.7/threading.py", line 551, in __bootstrap_inner
self.run()
File "/usr/lib/python2.7/threading.py", line 504, in run
self._target(*self.args, **self._kwargs)
File "/opt/couchbase/lib/python/pump.py", line 279, in run_worker
curx)
File "/opt/couchbase/lib/python/pump_bfd2.py", line 20, in check_spec
getattr(opts, "mode", "diff"))
File "/opt/couchbase/lib/python/pump_bfd.py", line 272, in find_seqno
for logpair in flogs:
TypeError: 'NoneType' object is not iterable
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