check_stale_header() fails to update the pointer to the previous header
while processing rdataset headers eligible for serve-stale, thus
enabling rdataset headers to be leaked (i.e. disassociated from a node
and left on the relevant TTL heap) while iterating through a node.  This
can lead to several different assertion failures.  Add the missing
pointer update.

(cherry picked from commit cf9cb7cd0013610c8978808aab9d6bae9fe57a4b)