Fixes crash where body was optional for PQ endpoint (it is not).

Fixes #3791

agent/prepared_query_endpoint.go

@@ -22,13 +22,11 @@ func (s *HTTPServer) preparedQueryCreate(resp http.ResponseWriter, req *http.Req
 	}
 	s.parseDC(req, &args.Datacenter)
 	s.parseToken(req, &args.Token)
-	if req.ContentLength > 0 {
 	if err := decodeBody(req, &args.Query, nil); err != nil {
 		resp.WriteHeader(http.StatusBadRequest)
 		fmt.Fprintf(resp, "Request decode failed: %v", err)
 		return nil, nil
 	}
-	}
 
 	var reply string
 	if err := s.agent.RPC("PreparedQuery.Apply", &args, &reply); err != nil {