sdsZmallocSize assumes a dynamically allocated SDS. When given a string
object created by createEmbeddedStringObject, it calls zmalloc_size on a
pointer that isn't the one returned by zmalloc

See issue #5250 and issue #5292 for more info.

Here the idea is that we do not want freeMemoryIfNeeded() to propagate a
DEL command before the script and change what happens in the script
execution once it reaches the slave. For example see this potential
issue (in the words of @soloestoy):

On master, we run the following script:

    if redis.call('get','key')
    then
        redis.call('set','xxx','yyy')
    end
    redis.call('set','c','d')

Then when redis attempts to execute redis.call('set','xxx','yyy'), we call freeMemoryIfNeeded(), and the key may get deleted, and because redis.call('set','xxx','yyy') has already been executed on master, this script will be replicated to slave.

But the slave received "DEL key" before the script, and will ignore maxmemory, so after that master has xxx and c, slave has only one key c.

Note that this patch (and other related work) was authored collaboratively in
issue #5250 with the help of @soloestoy and @oranagra.

Related to issue #5250.

See issue #5250 and the new comments added to the code in this commit
for details.

Technically speaking we don't really need to put the master client in
the clients that need to be processed, since in practice the PING
commands from the master will take care, however it is conceptually more
sane to do so.

However the master scripts will be impossible to kill.

Related to #5297.