internal/frontend: add missing return on invalid tab redirect

When a tab is invalid, the request is redirected to the main page. A
missing return is added.

Change-Id: I6db77dd456c275a70e68164c6ff75c12a87f012d
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/271417
Trust: Julie Qiu <julie@golang.org>
Run-TryBot: Julie Qiu <julie@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jamal Carvalho <jamal@golang.org>

internal/frontend/unit.go

@@ -83,6 +83,7 @@ func (s *Server) serveUnitPage(ctx context.Context, w http.ResponseWriter, r *ht
 	// Redirect to clean URL path when tab param is invalid.
 	if _, ok := unitTabLookup[tab]; !ok {
 		http.Redirect(w, r, r.URL.Path, http.StatusFound)
+		return nil
 	}
 
 	um, err := ds.GetUnitMeta(ctx, info.fullPath, info.modulePath, info.requestedVersion)