This variable was not actually used uninitialised, but some compilers
(e.g. gcc-4.3.3) warned that the variable might be used uninitialised.
Inspired by PR 41255 from Kurt Lidl.
Remove offset argument, we should now find an HFS+ volume in any
of its standard places.
Based on work from and test image provided by Pelle Johansson.
(hfslib_open_volume) We are only interested in the catalgo and
extents header, so read the first 512 bytes, not the whole first
extent. Also makes mounting a lot faster.
