fix crash in redis-cli after making cluster backup (#8267)

getRDB is "designed" to work in two modes: one for redis-cli --rdb and
one for redis-cli --cluster backup.
in the later case it uses the hiredis connection from the cluster nodes
and it used to free it without nullifying the context, so a later
attempt to free the context would crash.

I suppose the reason it seems to want to free the hiredis context ASAP
is that it wants to disconnect the replica link, so that replication
buffers will not be accumulated.