What's happening:
1. Query_cache::insert() locks the QC and verifies that it's enabled
2. parallel thread tries to disable it. trylock fails (QC is locked)
   so the status becomes DISABLE_REQUEST
3. Query_cache::insert() calls Query_cache::write_result_data()
   which allocates a new block and unlocks the QC.
4. Query_cache::unlock() notices there are no more QC users and a
   pending DISABLE_REQUEST so it disables the QC and frees all the
   memory, including the new block that was just allocated
5. Query_cache::write_result_data() proceeds to write into the freed block

Fix: change m_cache_status under a mutex.

Approved by Oleksandr Byelkin <sanja@mariadb.com>