Socket++ fork memory safery improvements

[MadVitaliy]

Right now an order of operations is:

1. Delete curr.
2. Access curr in "curr = curr->next"
3. Check if new curr is nullptr.

curr->next must be preserved before curr is deleted, what I have done in this PR. The flow now is:

1. If "curr" is nullptr nothing is done, loop is jut skipped (escaped).
2. "next" is preserved.
3. "curr" is deleted by the same condition as before, thus the behaviour should not be changed.
4. The preserve "next" is assigned to "curr"
5. Begin from 1.

[seanm]

The code change itself lgtm. But...

1. your explanation write-up is great, but would be nice to have in the actual commit message, not just in the github PR comments. (So that it's visible in git log and git history)
2. this is 3rd party code so ideally would be fixed upstream. I tried once to find the upstream, but I could not. @malaterre do you know if there is an upstream?

[MadVitaliy]

@seanm, I had a look on a possible original upstream. Malaterre's fork of it has a bit newer version than used in GDCM. And it turns out that the newer version has the same fix!

[seanm]

@seanm, I had a look on a possible original upstream. Malaterre's fork of it has a bit newer version than used in GDCM. And it turns out that the newer version has the same fix!

Would you be willing to diff GDCM's copy with that other repo and unify the two? Perhaps they each have fixes missing in the other...