Fix various places where code would work incorrectly if the common_header_len of events is different on master and slave

Patch developed with the help of Pavel Ivanov.

Also fix an uninitialised variable in queue_event().

sql/log.h

@@ -712,6 +712,7 @@ public:
   */
   bool appendv(const char* buf,uint len,...);
   bool append(Log_event* ev);
+  bool append_no_lock(Log_event* ev);
 
   void mark_xids_active(ulong cookie, uint xid_count);
   void mark_xid_done(ulong cookie, bool write_checkpoint);