gopls/internal/server: fix crash in SignatureHelp

Fix a crash when Snapshot.fileOf fails, due to e.g. context
cancellation. The defer of release should only occur after the error is
checked. Unfortunately, this is very hard to test since it likely occurs
only due to races.

This is our first bug found completely via automated crash reporting.

Fixes golang/go#66090

Change-Id: I7c22b5c2d1a0115718cd4bf2b84c31cc38a891ec
Reviewed-on: https://go-review.googlesource.com/c/tools/+/570675
Auto-Submit: Robert Findley <rfindley@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

gopls/internal/server/signature_help.go

@@ -19,10 +19,10 @@ func (s *server) SignatureHelp(ctx context.Context, params *protocol.SignatureHe
 	defer done()
 
 	fh, snapshot, release, err := s.fileOf(ctx, params.TextDocument.URI)
-	defer release()
 	if err != nil {
 		return nil, err
 	}
+	defer release()
 
 	if snapshot.FileKind(fh) != file.Go {
 		return nil, nil // empty result