Fix occasional incoherent back xpointer when following a link (#4061)
Fix occasional incoherent back xpointer when following a link (#4061)

In rare cases, the origin xpointer when following a link
was detected as incoherent, and not used when going back,
so no left margin marker was shown.
houqp/koreader,NiLuJe/koreader,Hzj-jie/koreader,koreader/koreader,mihailim/koreader,mwoz123/koreader,Markismus/koreader,Frenzie/koreader,NiLuJe/koreader,koreader/koreader,Frenzie/koreader,poire-z/koreader,poire-z/koreader,pazos/koreader