Correct flow memory usage bookkeeping error

Fix bug 1321 where flow_memuse was incremented more on allocation than
free.

Passes PR Scripts:
- PR build: https://buildbot.openinfosecfoundation.org/builders/ken-tilera/builds/53
- PR pcaps: https://buildbot.openinfosecfoundation.org/builders/ken-tilera-pcap/builds/59

Fixes bug 1321 - https://redmine.openinfosecfoundation.org/issues/1321