[Freeciv-Dev] (PR#12256) return of the unit_list_size assert

["Jason Short" <jdorje@xxxxxxxxxxxxxxxxxxxxx>]

<URL: http://bugs.freeciv.org/Ticket/Display.html?id=12256 >

1: 0x87c4248 Submarine at (56,46) ai2
civclient: packhand.c:1918: handle_tile_info: Assertion 
`unit_list_size(ptile->units) == 0' failed.
Aborted (core dumped)

I was sub-hunting with a submarine and the client crashed.

I believe i know why it happens, although it is not easy to reproduce.

- The enemy sub attacked and sunk one of my cruisers.  The server sends 
me info about the sub, of course.

- The death of the cruiser should have put the sub out of sight.  After 
the cruiser died the sub was out of vision of any of my units, although 
the tile the sub was on was still within my vision range.

- When I moved my unit that could see that tile out of range, the tile 
became fogged.  However the server didn't send a unit_goes_out_of_sight 
packet because the unit was supposed to already _be_ out of sight.

-jason