Fix core dump in contrib/xml2's xpath_table() when the input query returns a NULL value.  Per bug #4058.
Fix core dump in contrib/xml2's xpath_table() when the input query returns
a NULL value.  Per bug #4058.

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