Freeciv

Especially in a long game, units will get stuck in the
ACTIVITY_GOTO with a NULL goto_tile pointer; or stuck in ACTIVITY_IDLE
mode with a valid goto_tile pointer.

This fix does not address the how the units get it these degraded states,
 but it does either complete an interrupted ACTIVITY_GOTO (if possible), or
 return an ACTIVITY_GOTO_NOWHERE back to the ACTIVITY_IDLE mode, returning
 the unit to managed productivity.