Check for entity placement in random maps.

[Freagarach]

As pointed out by elexis, g_Map.placeEntityPassable doesn't always check if the entity is actually placed when assigning the return to a variable. In some cases the entity was not created but it is used later in the code, throwing an error.

It would be nice to check for such conditions.

[Freagarach]

In 24617:

Fix Jebel Barkal giant.

Adjusts the jb.js to not try and place walls outside of the map.
Only adds really added walls to the entity list (refs. #5919).

Patch by: @elexis
Differential revision: D3299
Tested by: @Freagarach, @Stan
Fixes: #5177
Refs. #5150