'flush tables' crashes

The server crashes when 'show procedure status' and 'flush tables' are
run concurrently.

This is caused by the way mysql.proc table is added twice to the list
of table to lock although the requirements on the current locking API
assumes differently.

No test case is submitted because of the nature of the crash which is 
currently difficult to reproduce in a deterministic way.

This is a backport from 5.1