CLEANUP: hpack: be careful about integer promotion from uint8_t

CLEANUP: hpack: be careful about integer promotion from uint8_t

As reported in issue #1635, there's a subtle sign change when shifting
a uint8_t value to the left because integer promotion first turns any
smaller type to signed int *even if it was unsigned*. A warning was
reported about uint8_t shifted left 24 bits that couldn't fit in int
due to this.

It was verified that the emitted code didn't change, as expected, but
at least this allows to silence the code checkers. There's no need to
backport this.