[syslinux] chain.c: fix v2 for public indexes in iterators (for master and disklib)

[Michal Soltys]

>From the earlier patches - that simple fix for indexes not counting non-data
partitions in gpt and mbr cases. Patches don't touch any other parts of the
code at this stage - bigger stuff later for disklib as we agreed - but this is
potentially useful right now.

Michal Soltys (1):
  chain.c: fix public index value in mbr and gpt iterators