Fix T43859: Texture baking crashes blender

The issue was caused by particles using billboard distribution, which
requires either object to be specified or camera in the scene to be
existing.

This commit prevents backing from crash, but bake result might not be
totally expected because of the thing mentioned above.

source/blender/render/intern/source/convertblender.c

@@ -1321,6 +1321,9 @@ static int render_new_particle_system(Render *re, ObjectRen *obr, ParticleSystem
 	if ((re->r.scemode & R_VIEWPORT_PREVIEW) && (ob->mode & OB_MODE_PARTICLE_EDIT))
 		return 0;
 
+	if (part->ren_as == PART_DRAW_BB && part->bb_ob == NULL && RE_GetCamera(re) == NULL)
+		return 0;
+
 /* 2. start initializing things */
 
 	/* last possibility to bail out! */