fixed an assert crash


include/mysql/plugin.h:
  more wl#2936 fixes: no implicit ha_thd() calls
mysql-test/mysql-test-run.pl:
  don't load system-wide plugins
mysql-test/r/partition_innodb.result:
  fix the test
mysql-test/t/partition_innodb.test:
  fix the test
sql/handler.cc:
  more wl#2936 fixes: no implicit ha_thd() calls
sql/handler.h:
  more wl#2936 fixes: no implicit ha_thd() calls
sql/sql_class.cc:
  more wl#2936 fixes: no implicit ha_thd() calls
sql/sql_plugin.cc:
  more wl#2936 fixes: assert crash
storage/innobase/handler/ha_innodb.cc:
  more wl#2936 fixes: no implicit ha_thd() calls