cubicweb-blog #3155851 fix prev/next adapter [validation pending]

When there is no blogentry (or if it is not traversable for e.g. security reasons), we should not just pick the immediate prev/next entity by eid...

prioritynormal
typebug
done in1.10.1
load0.100
load left0.000
closed by#8ba9dab9e67f fix prev_next adapter for BlogEntry
patchfix prev_next adapter for BlogEntry [applied]