During the test main.query_cache_innodb, only 16 bytes of
db_buf are initialized during the memcmp() in
dict_acquire_mdl_shared<false>(), but db_len was wrongly set to 20 bytes.

Something similar was fixed in MDEV-21344, but only for the table name,
in commit 0e25a8b4.

dict_table_t::parse_name(): Assign the return value of
filename_to_tablename() to the output parameters for lengths.
There is no need to invoke strlen().