HTTP/2: avoid left-shifting signed integer into the sign bit.

On non-aligned platforms, properly cast argument before left-shifting it in
ngx_http_v2_parse_uint32 that is used with u_char.  Otherwise it propagates
to int to hold the value and can step over the sign bit.  Usually, on known
compilers, this results in negation.  Furthermore, a subsequent store into a
wider type, that is ngx_uint_t on 64-bit platforms, results in sign-extension.

In practice, this can be observed in debug log as a very large exclusive bit
value, when client sent PRIORITY frame with exclusive bit set:

: *14 http2 PRIORITY frame sid:5 on 1 excl:8589934591 weight:17

Found with UndefinedBehaviorSanitizer.