Fix undefined behavior in avalanche.cpp

Summary:
In the MMIX LCG calculation, the right side calculation underlying type
is signed. This causes the undefined behavior sanitizer to detect a
signed integer overflow.
This diff sets the constants to unsigned long long to enforce the
underlying type to be unsigned and get rid of this error.

Test Plan:
With the UBSAN activated, run

test_bitcoin -t avalanche_tests

Reviewers: #bitcoin_abc, deadalnix, markblundeberg

Reviewed By: #bitcoin_abc, deadalnix, markblundeberg

Subscribers: markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3074