Fixes bug #20765: Stack overflow in animation because of weird WML files.

A call to unit::redraw_unit would check if the unit contains any animation. If
no animation is found, then it would call set_standing to put a new animation.

States are usually changed through unit::start_animation. unit::set_standing
uses this approach also. If unit::start_animation is unable to change the unit
to the animation passed as argument, it would try to animate the object calling
unit::set_standing (infinite recursion).

unit::start_animation had a check (maybe to prevent the infinite recursion), but
this check was buggy (if the intent was to prevent the infinite recursion).

The solution add a instruction to successfully set the unit to the state
STATE_STANDING and fixes the check that was using the wrong logical operator.