Fix bug introduced by MB-11259 [Buffer acknowledgment should have extlen of 4]

Update testapp so `make test` passes. Bug introduced by
b4a135a2e6c25c01dec9b09e51ca2ac6e838bcb8

Change-Id: I58e164a3670562d4ec0822ab6d566ae4f90d4e27
Reviewed-on: http://review.couchbase.org/38966
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>