http: fix undefined-shift in EVUTIL_IS*_ helpers

evutil.c:2559:1: runtime error: left shift of 1 by 31 places cannot be represented in type 'int'
    #0 0x4f2be0 in EVUTIL_ISXDIGIT_ libevent/evutil.c:2559:1
    #1 0x4bd689 in regname_ok libevent/http.c:4838:7
    #2 0x4bc16b in parse_authority libevent/http.c:4958:9
    #3 0x4bb8b5 in evhttp_uri_parse_with_flags libevent/http.c:5103:7
    #4 0x4bb762 in evhttp_uri_parse libevent/http.c:5050:9
    #5 0x4b8f41 in evhttp_parse_query_impl libevent/http.c:3505:9
    #6 0x4b8ed7 in evhttp_parse_query libevent/http.c:3569:9

Bug: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=23291
Report: https://oss-fuzz.com/testcase-detail/5670743106125824
(cherry picked from commit 37dbb350)