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