bugfix: federated/replication did not increment bytes_received status variable

because mysql->net.thd was reset to NULL in mysql_real_connect()
and thd_increment_bytes_received() didn't do anything.

Fix:
* set mysql->net.thd to current_thd instread.
* remove the test for non-null THD from a very often used
  function thd_increment_bytes_received().

include/my_pthread.h

@@ -367,6 +367,26 @@ int my_pthread_mutex_trylock(pthread_mutex_t *mutex);
 } while(0)
 #endif /* !set_timespec_time_nsec */
 
+#ifdef MYSQL_CLIENT
+#define _current_thd() NULL
+#elif defined(_WIN32)
+#ifdef __cplusplus
+extern "C"
+#endif
+MYSQL_THD _current_thd_noinline();
+#define _current_thd() _current_thd_noinline()
+#else
+/*
+  THR_THD is a key which will be used to set/get THD* for a thread,
+  using my_pthread_setspecific_ptr()/my_thread_getspecific_ptr().
+*/
+extern pthread_key(MYSQL_THD, THR_THD);
+static inline MYSQL_THD _current_thd(void)
+{
+  return my_pthread_getspecific_ptr(MYSQL_THD,THR_THD);
+}
+#endif
+
 /* safe_mutex adds checking to mutex for easier debugging */
 struct st_hash;
 typedef struct st_safe_mutex_t