Set fields to NULL after free in freeClient()

[QuChen88]

Null out several references after freeing the object in freeClient().

This is just to make the code more safe, to protect against use-after-free for future changes.

[hpatro]

Did it cause a double free or use-after-free somewhere we would need to fix?

[QuChen88]

I didn't spot an issue in OSS. This change is just to enforce some best practices of setting the pointer references to NULL after freeing the object to avoid future mistakes like such.

We did this change internally in our code base, so I figured it is better to open source it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.69%. Comparing base (9f4b1ad) to head (abd5f49).
Report is 169 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #1279      +/-   ##
============================================
+ Coverage     70.60%   70.69%   +0.09%     
============================================
  Files           112      114       +2     
  Lines         61533    63154    +1621     
============================================
+ Hits          43446    44648    +1202     
- Misses        18087    18506     +419     

Files with missing lines	Coverage Δ
src/networking.c	88.65% <100.00%> (-0.18%)	⬇️

... and 97 files with indirect coverage changes