util/disk_cache: fix stored_keys index

util/disk_cache: fix stored_keys index

It seems there is a bug because:
- 20 bytes are compared, but only 1 byte stored_keys step is used
- entries can overlap each other by 19 bytes
- index_mmap is ~1.3M in size, but only first 64K is used

With this fix for Deus Ex:
- startup time (from launch to Feral logo): ~38s -> ~16s
- disk_cache_has_key() hit rate: ~50% -> ~96%

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>