only close plugin server if actually created

[ndeloof]

- What I did
don't defer srv.Close() if srv was not successfully created (nil pointer)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 55.04%. Comparing base (9e50654) to head (b9c563a).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6123   +/-   ##
=======================================
  Coverage   55.03%   55.04%           
=======================================
  Files         361      361           
  Lines       30153    30152    -1     
=======================================
  Hits        16596    16596           
+ Misses      12599    12598    -1     
  Partials      958      958           

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[thaJeztah]

There's another call to srv.Close() further down; that one should probably be gated by a if srv != nil as well;

cli/cmd/docker/docker.go

Lines 331 to 337 in 9e50654

	// Terminate the plugin server, which will
	// close all connections with plugin
	// subprocesses, and signal them to exit.
	//
	// Repeated invocations will result in EINVAL,
	// or EBADF; but that is fine for our purposes.
	_ = srv.Close()

[thaJeztah]

LGTM, thanks!