MDEV-33534 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_double_round from sql/item_func.cc|

The negation in this line:
  ulonglong abs_dec= dec_negative ? -dec : dec;
did not take into account that 'dec' can be the smallest possible
signed negative value -9223372036854775808. Its negation is
an operation with an undefined behavior.

Fixing the code to use Longlong_hybrid, which implements a safe
method to get an absolute value.