ctx: Simplify handling for closed contexts.

[gdamore]

Once a context has started the process of close, further attempts to close it will return NNG_ECLOSED. What was I thinking to ever do anything else?

fixes #

Note that the above format should be used in your git commit comments.
You agree that by submitting a PR, you have read and agreed to our
contributing guidelines.

[codecov]

Codecov Report

Attention: Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.

Project coverage is 81.89%. Comparing base (4f940ce) to head (3dfa962).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/nng.c	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1972      +/-   ##
==========================================
+ Coverage   81.86%   81.89%   +0.03%     
==========================================
  Files          95       95              
  Lines       24074    24074              
  Branches     3209     3209              
==========================================
+ Hits        19707    19716       +9     
+ Misses       4295     4287       -8     
+ Partials       72       71       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.