merkle_tree:lookup/2: fix a too strong debug check
merkle_tree:lookup/2: fix a too strong debug check

(child list where I is a subset may also be empty)
scalaris-team/scalaris,jvf/scalaris,jvf/scalaris,caijieming-baidu/scalaris,scalaris-team/scalaris,caijieming-baidu/scalaris,caijieming-baidu/scalaris,scalaris-team/scalaris,scalaris-team/scalaris,caijieming-baidu/scalaris,caijieming-baidu/scalaris,scalaris-team/scalaris,scalaris-team/scalaris,scalaris-team/scalaris,caijieming-baidu/scalaris,jvf/scalaris,scalaris-team/scalaris,jvf/scalaris,jvf/scalaris,caijieming-baidu/scalaris,jvf/scalaris,jvf/scalaris