fix mroonga post-install script

it only worked if mroonga plugin wasn't installed before (normal case),
but then it didn't need to delete anything.

if, by some glitch, mroonga was already installed, it would delete
mroonga from mysql.plugin, but INSTALL would fail (as mroonga was running),
and the script aborted, leaving mroonga not in mysql.plugin at all.

storage/mroonga/data/install.sql.in

-DELETE IGNORE FROM mysql.plugin WHERE dl = 'ha_mroonga@MRN_PLUGIN_SUFFIX@';
-
-INSTALL PLUGIN Mroonga SONAME 'ha_mroonga@MRN_PLUGIN_SUFFIX@';
+SET @inst=IF(EXISTS(SELECT * FROM mysql.plugin WHERE NAME='mroonga'),'DO 1', "INSTALL PLUGIN mroonga SONAME 'ha_mroonga'");
+PREPARE s FROM @inst;
+EXECUTE s;
 
 DROP FUNCTION IF EXISTS last_insert_grn_id;
 CREATE FUNCTION last_insert_grn_id RETURNS INTEGER