Fix compilation warning in checkpoint.cc
Fix compilation warning in checkpoint.cc

" warning: using the result of an assignment as a condition without
 parentheses"

Change-Id: I9cb349996eb1da2c7cedb64b2b34a07be9f3939c
Reviewed-on: http://review.couchbase.org/45249
Reviewed-by: Sriram Ganesan <fde85c3124eed34938bb85159f430bf7e5b650f6@couchbase.com>
Tested-by: abhinav dangeti <d73ef2279de6577f7bf081737ee47554bbb7dc82@couchbase.com>

hisundar/ep-engine,hisundar/ep-engine,sriganes/ep-engine,daverigby/kv_engine,sriganes/ep-engine,daverigby/ep-engine,sriganes/ep-engine,membase/ep-engine,daverigby/ep-engine,owendCB/ep-engine,membase/ep-engine,daverigby/ep-engine,teligent-ru/ep-engine,couchbase/ep-engine,owendCB/ep-engine,couchbase/ep-engine,owendCB/ep-engine,jimwwalker/ep-engine,daverigby/kv_engine,jimwwalker/ep-engine,teligent-ru/ep-engine,teligent-ru/ep-engine,daverigby/kv_engine,owendCB/ep-engine,hisundar/ep-engine,couchbase/ep-engine,membase/ep-engine,membase/ep-engine,couchbase/ep-engine,hisundar/ep-engine,sriganes/ep-engine,daverigby/kv_engine,daverigby/ep-engine,jimwwalker/ep-engine,jimwwalker/ep-engine,teligent-ru/ep-engine