Added a test case for bug #6447.
mf_keycache.c:
  Fixed bug #6447. Erronious code in the key_cache_read function
  caused problems when reading key blocks with offset>0 into
  key cache. The code of key_cache_insert and key_cache_write
  was modified similarly.


mysys/mf_keycache.c:
  Fixed bug #6447. Erronious code in the key_cache_read function
  caused problems when reading key blocks with offset>0 into
  key cache. The code of key_cache_insert and key_cache_write
  was modified similarly.
mysql-test/t/key_cache.test:
  Added a test case for bug #6447.
mysql-test/r/key_cache.result:
  Added a test case for bug #6447.