mod_http2: Fix warnings:

modules/http2/h2_util.c|174| warning: suggest parentheses around ‘+’ in operand of ‘&’
modules/http2/h2_util.c|176| warning: suggest parentheses around ‘+’ in operand of ‘&’

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1726798 13f79535-47bb-0310-9956-ffa450edef68