gopls/internal/golang/completion: avoid crash in addFieldItems

For now, be defensive and avoid the crash reported in golang/go#72828.
No attempt was made to reproduce.

Longer term, as the TODO indicates, we should investigate the logic
error that leads to addFieldItems being called with nil surrounding.

Fixes golang/go#72828

Change-Id: I2300406b49fc3d53561b288d42f64793429e3fbd
Reviewed-on: https://go-review.googlesource.com/c/tools/+/659237


Reviewed-by: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Findley <rfindley@google.com>