Fix crash in mnesia_controller due to a function_clause exception from is_tab_blocked/1

If mnesia_controller:block_table/1 is called twice for same table,
the new stored value of {Tab, where_to_commit} will be {blocked, {blocked,List}}.
This will result in an exception when calling is_tab_blocked/1.
mnesia_conttoller:block_table/1 should check if the table is already blocked.
Merge branch 'fix_mnesia_block_table' of https://github.com/spawnthink/otp into fix_mnesia_block_table

Conflicts:
	lib/mnesia/src/mnesia_controller.erl