sumdb: add missing return after http.Error

Change-Id: Ic0a9149713f7981e44629b2d96cf2a6ec52d6976
Reviewed-on: https://go-review.googlesource.com/c/mod/+/500455
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

sumdb/server.go

@@ -148,6 +148,7 @@ func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 				msg, err := tlog.FormatRecord(start+int64(i), text)
 				if err != nil {
 					http.Error(w, err.Error(), http.StatusInternalServerError)
+					return
 				}
 				data = append(data, msg...)
 			}