LP #1729241: Build failure on Ubuntu artful

**Reported in Launchpad by Sergei Glushchenko last update 30-11-2017 21:44:47

[ 92%] Building CXX object sql/CMakeFiles/sql.dir/string_service.cc.o
[ 92%] Building CXX object sql/CMakeFiles/sql.dir/sql_acl.cc.o
/root/percona-xtrabackup/sql/sql_acl.cc: In function int replace_user_table(THD*, TABLE*, LEX_USER*, ulong, bool, bool, bool):
/root/percona-xtrabackup/sql/sql_acl.cc:3017:59: error: ISO C++ forbids comparison between pointer and integer [-fpermissive]
if (combo->plugin.str == NULL || combo->plugin.str == '\0')
^~~~
sql/CMakeFiles/sql.dir/build.make:1613: recipe for target 'sql/CMakeFiles/sql.dir/sql_acl.cc.o' failed
make[2]: *** [sql/CMakeFiles/sql.dir/sql_acl.cc.o] Error 1
CMakeFiles/Makefile2:3874: recipe for target 'sql/CMakeFiles/sql.dir/all' failed
make[1]: *** [sql/CMakeFiles/sql.dir/all] Error 2
Makefile:151: recipe for target 'all' failed
make: *** [all] Error 2