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

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