fix problem

Error: Freeing unallocated data at line 900, 'sql_db.cc'
(
 DROP DATABASE frees thd->db for everything but thd->slave_thread=1
 however we are not slave thread and more bugs could appear if we 
 pretend to be, then we will be good boys and just strdup the value.
 If not DROP DATABASE then THD::~THD() will free the value.
)

sql/event_timed.cc

@@ -1156,7 +1156,12 @@ Event_timed::execute(THD *thd, MEM_ROOT *mem_root)
     goto done;
   /* Now we are sure we have valid this->sphead so we can copy the context */
   sphead->m_security_ctx= security_ctx;
-  thd->db= dbname.str;
+  /*
+    THD::~THD will clean this or if there is DROP DATABASE in the SP then
+    it will be free there. It should not point to our buffer which is allocated
+    on a mem_root.
+  */
+  thd->db= my_strdup(dbname.str, MYF(0));
   thd->db_length= dbname.length;
   if (!check_access(thd, EVENT_ACL,dbname.str, 0, 0, 0,is_schema_db(dbname.str)))
   {
@@ -1173,7 +1178,6 @@ Event_timed::execute(THD *thd, MEM_ROOT *mem_root)
   restore_security_context(thd, save_ctx);
   DBUG_PRINT("info", ("master_access=%d db_access=%d",
              thd->security_ctx->master_access, thd->security_ctx->db_access));
-  thd->db= 0;
 
   VOID(pthread_mutex_lock(&this->LOCK_running));
   running= false;
@@ -1236,12 +1240,12 @@ Event_timed::change_security_context(THD *thd, Security_context *s_ctx,
     Event_timed::restore_security_context()
       thd    - thread
       backup - switch to this context
- */
+*/
 
 void
 Event_timed::restore_security_context(THD *thd, Security_context *backup)
 {
-  DBUG_ENTER("Event_timed::change_security_context");
+  DBUG_ENTER("Event_timed::restore_security_context");
 #ifndef NO_EMBEDDED_ACCESS_CHECKS
   if (backup)
     thd->security_ctx= backup;
@@ -1489,7 +1493,6 @@ Event_timed::spawn_thread_finish(THD *thd)
     1 - not locked by this thread
 */
 
-
 int
 Event_timed::spawn_unlock(THD *thd)
 {