plugins: don't panic on Close if PluginServer nil

[laurazard]

fixes#5317

- What I did

We had a report in fixes#5317 of a panic at

cli/cli-plugins/socket/socket.go

Line 111 in 87c6624

pl.mu.Lock()

Looks like this will happen if the call to create a PluginServer fails and returns nil, since we're not checking for nilness before attempting to call Close:

cli/cmd/docker/docker.go

Lines 247 to 251 in 87c6624

	deferfunc() {
	// Close the server when plugin execution is over, so that in case
	// it's still open, any sockets on the filesystem are cleaned up.
	_=srv.Close()
	}()

cli/cmd/docker/docker.go

Line 278 in 87c6624

_=srv.Close()

- How I did it

Added a nil check to PluginServer.Close(). We could also have addressed this at the caller sites, but this seems more robust for little cost.

- How to verify it

- Description for the changelog

Fixed issue where main CLI process would sometimes panic while running a plugin.

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.45%. Comparing base (87c6624) to head (9c44806).

Additional details and impacted files

@@ Coverage Diff @@## master #5327 +/- ##
=======================================
Coverage 61.45% 61.45% =======================================
Files 299 299 Lines 20855 20857 +2 =======================================
+ Hits 12816 12818 +2 
Misses 7124 7124 Partials 915 915 

[laurazard]

Wondering if we should also backport this to 26.1.

[thaJeztah]

LGTM

[thaJeztah]

Wondering if we should also backport this to 26.1.

It's probably ok to cherry-pick; we may not be doing releases from that branch, but a cherry-pick doesn't do harm