MB-11104 Fix to a bug in identifying non-existent keys in batch read

This bug causes the batch read to return key_not_found error to
the caller even for existing keys.
Change-Id: I2fb445b41ed04e2f091b2e291c20facf68301f7e
Reviewed-on: http://review.couchbase.org/37496
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>