Fix to bug #463, http://bugs.skysql.com/show_bug.cgi?id=463

mysql_common.c:gw_MySQL_get_next_packet didn't handle case where an insert command followed by alter table in the same read buffer. It shouldn't been possible without multi-statement being set.

server/core/dcb.c

@@ -814,6 +814,8 @@ int	below_water;
         }
                 
         spinlock_acquire(&dcb->writeqlock);
+        
+        ss_dassert(dcb->state != DCB_STATE_ZOMBIE);
 
 	if (dcb->writeq != NULL)
 	{