ekanalyzer frushow: Fix internal area size calc

When there was no chassis and board areas, the internal use
are size was calculated incorrectly.

Additionally, it is possible that the internal area is actually
located after all other areas or that areas are arranged in
a different order than their offsets in the header. All those
cases weren't properly handled.

Now that is fixed.

Signed-off-by: Alexander Amelkin <alexander@amelkin.msk.ru>