Fix incorrect assertion in client list operations

[uriyage]

The current assertion introduced in redis/redis#11220

serverAssert(&c->clients_pending_write_node.next != NULL || &c->clients_pending_write_node.prev != NULL);

is incorrect for two reasons:

1. Using &pointer.next would always be non-NULL since it's the address of the field.
2. The check is incorrect even without the & because in a single-node list, both pointers can be NULL.

Fix:

1. Remove the always-true assertion
2. Add proper assertions in listUnlinkNode to ensure the node membership in the list to cover all list cases.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 71.00%. Comparing base (677d575) to head (35ad39f).
Report is 4 commits behind head on unstable.

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable    #1800      +/-   ##
============================================
- Coverage     71.17%   71.00%   -0.18%     
============================================
  Files           123      123              
  Lines         65648    65655       +7     
============================================
- Hits          46724    46617     -107     
- Misses        18924    19038     +114     

Files with missing lines	Coverage Δ
src/adlist.c	77.50% <100.00%> (+0.57%)	⬆️
src/networking.c	88.77% <ø> (-0.01%)	⬇️

... and 20 files with indirect coverage changes

[zuiderkwast]

Good! Small comment though.