RB: set the new read pointer after clearing the header.
RB: set the new read pointer after clearing the header.

This is to prevent a situation where a fast writer will
write their new chunk between setting the new read pointer
and clearing the header.

Signed-off-by: Angus Salkeld <86b65304d27d8de73dd7d624c33df7e088f8d94b@redhat.com>

kgaillot/libqb,rubenk/libqb,gao-yan/libqb,ClusterLabs/libqb,ClusterLabs/libqb,davidvossel/libqb,rubenk/libqb,ClusterLabs/libqb,davidvossel/libqb,kgaillot/libqb,gao-yan/libqb,kgaillot/libqb