Don't allow one to kill START SLAVE while the slaves IO_THREAD or SQL_THREAD

 is starting.

This is needed as if we kill the START SLAVE thread too early during
shutdown then the IO_THREAD or SQL_THREAD will not have time to properly
initlize it's replication or THD structures and clean_up() will try to
delete master_info structures that are still in use.

sql/slave.cc

@@ -831,6 +831,15 @@ int start_slave_thread(
       mysql_mutex_unlock(start_lock);
     DBUG_RETURN(ER_SLAVE_THREAD);
   }
+
+  /*
+    In the following loop we can't check for thd->killed as we have to
+    wait until THD structures for the slave thread are created
+    before we can return.
+    This should be ok as there is no major work done in the slave
+    threads before they signal that we can stop waiting.
+  */
+
   if (start_cond && cond_lock) // caller has cond_lock
   {
     THD* thd = current_thd;
@@ -848,16 +857,9 @@ int start_slave_thread(
         registered, we could otherwise go waiting though thd->killed is
         set.
       */
-      if (!thd->killed)
-        mysql_cond_wait(start_cond, cond_lock);
+      mysql_cond_wait(start_cond, cond_lock);
       thd->EXIT_COND(& saved_stage);
       mysql_mutex_lock(cond_lock); // re-acquire it as exit_cond() released
-      if (thd->killed)
-      {
-        if (start_lock)
-          mysql_mutex_unlock(start_lock);
-        DBUG_RETURN(thd->killed_errno());
-      }
     }
   }
   if (start_lock)