Fix WAL file cutoff point calculation in pg_standby.
Fix WAL file cutoff point calculation in pg_standby.

Patch by Simon Riggs, per bug report from Ferenc Felhoffer

Quikling/gpdb,lintzc/gpdb,CraigHarris/gpdb,chrishajas/gpdb,xinzweb/gpdb,50wu/gpdb,50wu/gpdb,CraigHarris/gpdb,ashwinstar/gpdb,edespino/gpdb,greenplum-db/gpdb,rvs/gpdb,janebeckman/gpdb,chrishajas/gpdb,Quikling/gpdb,Quikling/gpdb,lintzc/gpdb,ashwinstar/gpdb,kaknikhil/gpdb,lintzc/gpdb,50wu/gpdb,0x0FFF/gpdb,kaknikhil/gpdb,jmcatamney/gpdb,0x0FFF/gpdb,cjcjameson/gpdb,zaksoup/gpdb,lintzc/gpdb,adam8157/gpdb,xinzweb/gpdb,jmcatamney/gpdb,lintzc/gpdb,greenplum-db/gpdb,Quikling/gpdb,Chibin/gpdb,yuanzhao/gpdb,ahachete/gpdb,yuanzhao/gpdb,zaksoup/gpdb,zaksoup/gpdb,yuanzhao/gpdb,ahachete/gpdb,janebeckman/gpdb,0x0FFF/gpdb,Quikling/gpdb,greenplum-db/gpdb,Quikling/gpdb,0x0FFF/gpdb,cjcjameson/gpdb,ashwinstar/gpdb,cjcjameson/gpdb,adam8157/gpdb,jmcatamney/gpdb,kaknikhil/gpdb,cjcjameson/gpdb,lisakowen/gpdb,xuegang/gpdb,chrishajas/gpdb,yuanzhao/gpdb,ashwinstar/gpdb,zaksoup/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,lisakowen/gpdb,yuanzhao/gpdb,greenplum-db/gpdb,lintzc/gpdb,royc1/gpdb,kaknikhil/gpdb,cjcjameson/gpdb,rvs/gpdb,0x0FFF/gpdb,lisakowen/gpdb,chrishajas/gpdb,edespino/gpdb,yuanzhao/gpdb,lisakowen/gpdb,ahachete/gpdb,Chibin/gpdb,xuegang/gpdb,Quikling/gpdb,Chibin/gpdb,edespino/gpdb,yuanzhao/gpdb,janebeckman/gpdb,royc1/gpdb,ahachete/gpdb,Chibin/gpdb,lisakowen/gpdb,adam8157/gpdb,greenplum-db/gpdb,ahachete/gpdb,50wu/gpdb,adam8157/gpdb,zaksoup/gpdb,royc1/gpdb,xuegang/gpdb,Quikling/gpdb,rvs/gpdb,rvs/gpdb,zaksoup/gpdb,xinzweb/gpdb,jmcatamney/gpdb,yuanzhao/gpdb,xuegang/gpdb,chrishajas/gpdb,adam8157/gpdb,kaknikhil/gpdb,Chibin/gpdb,50wu/gpdb,cjcjameson/gpdb,royc1/gpdb,ashwinstar/gpdb,janebeckman/gpdb,0x0FFF/gpdb,edespino/gpdb,lisakowen/gpdb,lisakowen/gpdb,rvs/gpdb,ahachete/gpdb,50wu/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,chrishajas/gpdb,edespino/gpdb,royc1/gpdb,zaksoup/gpdb,Chibin/gpdb,greenplum-db/gpdb,adam8157/gpdb,kaknikhil/gpdb,janebeckman/gpdb,xuegang/gpdb,lisakowen/gpdb,royc1/gpdb,xuegang/gpdb,kaknikhil/gpdb,ashwinstar/gpdb,kaknikhil/gpdb,rvs/gpdb,jmcatamney/gpdb,Chibin/gpdb,xinzweb/gpdb,0x0FFF/gpdb,xinzweb/gpdb,janebeckman/gpdb,ashwinstar/gpdb,kaknikhil/gpdb,edespino/gpdb,xuegang/gpdb,janebeckman/gpdb,cjcjameson/gpdb,janebeckman/gpdb,CraigHarris/gpdb,xinzweb/gpdb,cjcjameson/gpdb,royc1/gpdb,xinzweb/gpdb,lintzc/gpdb,CraigHarris/gpdb,rvs/gpdb,edespino/gpdb,50wu/gpdb,CraigHarris/gpdb,50wu/gpdb,janebeckman/gpdb,rvs/gpdb,xinzweb/gpdb,edespino/gpdb,rvs/gpdb,yuanzhao/gpdb,adam8157/gpdb,edespino/gpdb,xuegang/gpdb,Chibin/gpdb,adam8157/gpdb,xuegang/gpdb,0x0FFF/gpdb,CraigHarris/gpdb,kaknikhil/gpdb,Quikling/gpdb,chrishajas/gpdb,jmcatamney/gpdb,ahachete/gpdb,janebeckman/gpdb,edespino/gpdb,Chibin/gpdb,zaksoup/gpdb,lintzc/gpdb,chrishajas/gpdb,Quikling/gpdb,ahachete/gpdb,royc1/gpdb,lintzc/gpdb,rvs/gpdb,cjcjameson/gpdb,yuanzhao/gpdb,CraigHarris/gpdb,cjcjameson/gpdb,CraigHarris/gpdb,Chibin/gpdb,jmcatamney/gpdb,CraigHarris/gpdb