dfs: Fix DataFormatException: 0 bytes to inflate

dfs: Fix DataFormatException: 0 bytes to inflate

When a file uses a different block size (e.g.  500) than the cache
(e.g.  512), and the DfsPackFile's blockSize field has not been
initialized, the cache misaligns block loads.  The cache uses its
default of 512 to compute the block alignment instead of the file's
500.

This causes DfsReader try to set an empty range into an Inflater,
resulting in an object being unable to load.

Change-Id: I7d6352708225f62ef2f216d1ddcbaa64be113df6