Assert: "tree_index >= 0" after #1630 when model with $nocollide_this_only explodes

[tomimaki]

Release build gives me multiple times

and debug build: ASSERTION: "tree_index >= 0" at modelread.cpp:5232
fs2_open.log

Example is Shivan Commnode from Mediavps 2014. Load mission Into Lion's Den (loop2-2.fs2), pack Trebuchet, destroy Crystal in Commnode and wait until it explode.

Another example is Vishnan_Protector from BP but can't find reliable mission.

[ngld]

The debug assertion is caused by the fact that submodel_get_two_random_points_better assumes that detail0 always has a collision tree. If the tree hasn't been loaded, the tree_index is -1 which then causes that assertion in model_get_bsp_collision_tree.

A nice solution would be to make submodel_get_two_random_points_better use a collide subobject if one is available instead of detail0 but I don't know enough about the code to do this myself.

[Goober5000]

I don't know about collision subobjects myself, but it's a simple matter to fall back to the old method, so I made a new PR to do just that.

[tomimaki]

I can confirm that #1733 fixed issue.