[PATCH] Storage plugin can caush segfault in table.cc init_tmp_table_share()

(Context: I'm working with Arjen Lentz on opengraph )

If a plugin passes a NULL tablename to init_tmp_table_share() this causes a segfault.

If I assume that the core mysqld should be robust, this should be trapped (although not necessarily able to prevent a related segfault elsewhere in the storage plugin)

I have attached a patch that deals with this, by checking the arguments that are otherwise dereferenced by strlen() before it is used and either ASSERTs in debug build or returning with a warning log.