MDEV-31221 UBSAN runtime error: negation of -9223372036854775808 cannot be represented in type 'long long int' in my_strtoll10_utf32

The code in my_strtoll10_mb2 and my_strtoll10_utf32
could hit undefinite behavior by negation of LONGLONG_MIN.
Fixing to avoid this.

Also, fixing my_strtoll10() in the same style.
The previous reduction produced a redundant warning on
CAST(_latin1'-9223372036854775808' AS SIGNED)