Add check in CLUSTERLINK KILL cmd to avoid freeing links to myself

[pieturin]

Add check in CLUSTERLINK KILL cmd to avoid freeing cluster bus links to myself. Also add an assert in freeClusterLink().

Testing:

127.0.0.1:6379> debug clusterlink kill all c0404ee68574c6aa1048aaebfe90283afe51d2fc
(error) ERR Cannot free cluster link(s) to myself

[madolson]

@pieturin Do you want to add some more information about why you added the assert? The debug check seems fine, since the node will exist but it won't have any links.

[pieturin]

Adding the assert should avoid more errors like the one fixed by this commit for example.

[hpatro]

LGTM.

[madolson]

OK, the server assert seems like a good defense to make sure we aren't accessing random memory.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 70.03%. Comparing base (32ca6e5) to head (ccd9108).
Report is 3 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable     #689      +/-   ##
============================================
- Coverage     70.17%   70.03%   -0.14%     
============================================
  Files           110      110              
  Lines         60092    60104      +12     
============================================
- Hits          42170    42096      -74     
- Misses        17922    18008      +86     

Files	Coverage Δ
src/cluster_legacy.c	85.81% <50.00%> (-0.19%)	⬇️

... and 12 files with indirect coverage changes