HTTP/3: fixed overflow in prefixed integer parser.

HTTP/3: fixed overflow in prefixed integer parser.

Previously, the expression (ch & 0x7f) was promoted to a signed integer.
Depending on the platform, the size of this integer could be less than 8 bytes,
leading to overflow when handling the higher bits of the result.  Also, sign
bit of this integer could be replicated when adding to the 64-bit st->value.