MDEV-28386 UBSAN: runtime error: negation of -X cannot be represented in type 'long long int'; cast to an unsigned type to negate this value to itself in my_strntoull_8bit on SELECT ... OCT

The code in my_strntoull_8bit() and my_strntoull_mb2_or_mb4()
could hit undefinite behavior by negating of LONGLONG_MIN.
Fixing the code to avoid this.