* src/augeas.c (tree_replace): don't bother marking the new parent as dirty
* src/augeas.c (tree_replace): don't bother marking the new parent as dirty

We call tree_rm on it earlier, which already marks it as dirty.

This line also triggered a SEGV when sub == NULL; this happened, for
example, when /etc/apt/preferences was an empty file.

Bug reported by Matt Palmer

kunkku/augeas,raphink/augeas,ptoscano/augeas,GeoffWilliams/augeas,dafugg/augeas,pevalme/augeas,hercules-team/augeas-do-not-use,MikaelSmith/augeas,hercules-team/augeas,jasperla/augeas,raphink/augeas,mchf/augeas,pevalme/augeas,kumy/augeas,jtopjian/augeas,jjlin/augeas,mlichvar/augeas,ptoscano/augeas,pevalme/augeas,ptoscano/augeas,manandbytes/augeas,domcleal/augeas,manandbytes/augeas,domcleal/augeas,lutter/augeas,kunkku/augeas,kumy/augeas,kumy/augeas,kunkku/augeas,hercules-team/augeas,jtopjian/augeas,MikaelSmith/augeas,kumy/augeas,jasperla/augeas,dafugg/augeas,domcleal/augeas,lutter/augeas,jjlin/augeas,mchf/augeas,jjlin/augeas,dafugg/augeas,mlichvar/augeas,lutter/augeas,hercules-team/augeas-do-not-use,mlichvar/augeas,hercules-team/augeas-do-not-use,GeoffWilliams/augeas