A crash was observed where dict_acquire_mdl_shared<trylock=false>
would invoke memcpy() with an apparently uninitialized tbl_len.

dict_table_t::parse_name(): Remove an unnecessary tbl_len--
operation. (This should be mostly non-functional cleanup.)

dict_acquire_mdl_shared(): If the second dict_table_t::parse_name()
returns false, terminate the loop just like we would do on the
first invocation.