BUG/MINOR: qpack: missing shift count check in qpack_get_varint() (UB)

BUG/MINOR: qpack: missing shift count check in qpack_get_varint() (UB)

Parsing QPACK varints with excessive continuation bytes increases shift
by 7 on each iteration. When shift reaches 63 or higher, evaluating:

    (v & 127) > (limit - ret) >> shift

executes a bitwise shift on a 64-bit integer with an out-of-range count,
which is Undefined Behavior in C.

Fix this by jumping to too_large label if shift >= 63 before the shift runs.

Thank you to @OneZ3r0 for having reported this issue.

Should be backported to all versions.