Bug fix as reported by Fish-Face/Chris Le Sueur on IRC

After deleting an object, the call free_and_unlink_base() takes care of
NULLifying object pointers. But, with exception of constraints. Silly!!!

So, in the end of a function additionally the test_scene_constraints()
has to be called to prevent crashes. This was missing in join_mesh and
convert_object.

source/blender/src/meshtools.c

@@ -432,6 +432,8 @@ void join_mesh(void)
 	enter_editmode();
 	exit_editmode(1);	// freedata, but no undo
 	
+	test_scene_constraints(); // always call after delete object (stupid!)
+
 	allqueue(REDRAWVIEW3D, 0);
 	allqueue(REDRAWBUTSSHADING, 0);
 	makeDispList(ob);