Merge branch 'dr/midx-avoid-int-underflow'

When fed a midx that records no objects, some codepaths tried to
loop from 0 through (num_objects-1), which, due to integer
arithmetic wrapping around, made it nonsense operation with out of
bounds array accesses.  The code has been corrected to reject such
an midx file.

* dr/midx-avoid-int-underflow:
  midx.c: fix an integer underflow