internal/lsp: handle potential nil pointer

Change-Id: I345b0d22053248f875683d87515d9e7c5e1d16c6
Reviewed-on: https://go-review.googlesource.com/c/tools/+/195518
Reviewed-by: Ian Cottrell <iancottrell@google.com>
Run-TryBot: Rebecca Stambler <rstambler@golang.org>

internal/lsp/source/util.go

@@ -123,6 +123,9 @@ func pkgToMapper(ctx context.Context, view View, pkg Package, uri span.URI) (*as
 			ph = h
 		}
 	}
+	if ph == nil {
+		return nil, nil, errors.Errorf("no ParseGoHandle for %s", uri)
+	}
 	file, err := ph.Cached(ctx)
 	if file == nil {
 		return nil, nil, err