trace_mpath: fix clear_infection/restore_infection (bug since r5599).
trace_mpath: fix clear_infection/restore_infection (bug since r5599).

When clear_- and restore_infection are called several times in
sequence and the infection is gone in between, an outdated infection
was restored by restore_infection. By deleting the backup when
clear_infection does not find an infection, the backup is kept up to
date, which fixes the misbehaviour.


jvf/scalaris,caijieming-baidu/scalaris,jvf/scalaris,caijieming-baidu/scalaris,caijieming-baidu/scalaris,caijieming-baidu/scalaris,caijieming-baidu/scalaris,scalaris-team/scalaris,caijieming-baidu/scalaris,scalaris-team/scalaris,scalaris-team/scalaris,jvf/scalaris,jvf/scalaris,scalaris-team/scalaris,jvf/scalaris,jvf/scalaris,scalaris-team/scalaris,scalaris-team/scalaris,caijieming-baidu/scalaris,scalaris-team/scalaris,scalaris-team/scalaris,jvf/scalaris