[freeciv-ai] (PR#4018) Virtual unit in CHECK_UNIT

["Gregory Berkolaiko" <Gregory.Berkolaiko@xxxxxxxxxxxx>]

Here is an excerpt from backtrace:

#3  0x42022ecb in __assert_fail () from /lib/i686/libc.so.6
#4  0x080c179e in find_beachhead (punit=0xbfffeea0, dest_x=20,
dest_y=20, x=0xbfffee14, y=0xbfffee18)
    at aiunit.c:1239
#5  0x080b6451 in assess_distance (pcity=0x8398980, punit=0xbfffeea0,
move_rate=3, boatid=151,
    boatdist=255, boatspeed=12) at advmilitary.c:329
#6  0x080b691b in assess_danger (pcity=0x8398980) at advmilitary.c:478

Essentially, AI constructs a virtual unit in assess_danger, if the enemy
is going to build it next turn.  Then tries to estimate the distance, in
assess_distance.  If boatid != 0, assess_distance calls find_beachhead,
which attempts to verify the unit.

Easiest solution: kill the assert.

G.