- fix for particle related crash in editmode. particle system calculation

is pretty haphazard at the moment, needs to be reviewed before next
   release (as does duplis).

source/blender/blenkernel/intern/effect.c

@@ -1261,7 +1261,16 @@ void build_particle_system(Object *ob)
 	}
 	
 	/* init */
+	
+		/* this call returns NULL during editmode, just ignore it and
+		 * particles should be recalc'd on exit.
+		 */
 	dm = mesh_get_derived_final(ob, &dmNeedsFree);
+	if (!dm) {
+		waitcursor(0);
+		return;
+	}
+
 	dlm = dm->convertToDispListMesh(dm);
 	totvert = dlm->totvert;