Merge branch 'sn/null-pointer-arith-in-mark-tree-uninteresting'

mark_tree_uninteresting() has code to handle the case where it gets passed a NULL pointer in its 'tree' parameter, but the function had 'object = &tree->object' assignment before checking if tree is NULL. This gives a compiler an excuse to declare that tree will never be NULL and apply a wrong optimization. Avoid it. * sn/null-pointer-arith-in-mark-tree-uninteresting: revision.c: fix possible null pointer arithmetic