When concurrent inserts were disabled, statements after an INSERT
were not put into the query cache. This happened because we do not
save the current data file length at statement start when
concurrent inserts are disabled. But we checked the always zero
local length against the real file length anyway.
  
Fixed by doing the check only if concurrent inserts are not diabled.


mysql-test/r/query_cache.result:
  Bug#33756 - query cache with concurrent_insert=0 appears broken
  Added test result.
mysql-test/t/query_cache.test:
  Bug#33756 - query cache with concurrent_insert=0 appears broken
  Added test.
sql/ha_myisam.cc:
  Bug#33756 - query cache with concurrent_insert=0 appears broken
  Changed code so that file length check is done only when
  concurrent inserts are possible.