gopls/internal/golang: add missing ,ok check

Fixes golang/go#74058

Change-Id: Ia128f224fc4ca7326320e43b1e485f846d50b846
Reviewed-on: https://go-review.googlesource.com/c/tools/+/680257
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>

Author: adonovan

gopls/internal/golang/hover.go

@@ -356,8 +356,8 @@ func hover(ctx context.Context, snapshot *cache.Snapshot, fh file.Handle, pp pro
 	}
 	if obj, ok := obj.(*types.Var); ok && obj.IsField() {
 		if selExpr, ok := cur.Parent().Node().(*ast.SelectorExpr); ok {
-			sel := pkg.TypesInfo().Selections[selExpr]
-			if len(sel.Index()) > 1 {
+			sel, ok := pkg.TypesInfo().Selections[selExpr]
+			if ok && len(sel.Index()) > 1 {
 				var buf bytes.Buffer
 				buf.WriteString(" // through ")
 				t := typesinternal.Unpointer(sel.Recv())