MDEV-33729 UBSAN negation of -X cannot be represented in type 'long long int'; cast to an unsigned type to negate this value to itself in my_strntoll_mb2_or_mb4

The problem was introduced by MDEV-30879.
The function my_strntoll_8bit() was correctly changed by MDEV-30879.
The function my_strntoll_mb2_or_mb4() was not.

Applying the missing change to my_strntoll_mb2_or_mb4().