Verifier handles large negative unsigned values incorreclty

[Alan-Jowett]

Program Counter: 3
Registers: r0=8000000000004000
Label: 3:-1
Concrete state does not match invariant
--- Concrete state ---
[
    packet_size=3,
    r0.svalue=-9223372036854759424, r0.uvalue=9223372036854792192]
Stack: Numbers -> {}
--- Abstract state ---
[
    meta_offset=0,
    packet_size=[0, 65534],
    r0.svalue=9223372036854792192, r0.type=number, r0.uvalue=9223372036854792192,
    r1.ctx_offset=0, r1.svalue=[1, 2147418112], r1.type=ctx,
    r10.stack_offset=512, r10.svalue=[512, 2147418112], r10.type=stack]
Stack: Numbers -> {}

Correct int64_t representation of 8000000000004000 is -9223372036854759424, not 9223372036854792192.

[Alan-Jowett]

Fixed by #769