unpack_sha1_file issues

unpack_sha1_file is being called by fsck-cache.  Therefore it should
not assume that
a \0 occurs within the first 8192 bytes of the uncompressed data. 
However, currently
both the sscanf and strlen calls do just that.

Also, unpack_sha1_file should call inflateEnd in a couple of the error cases.

Finally, if *size==0, shouldn't unpack_sha1_file allocate a single
byte to prevent
malloc from returning NULL?

Morten