Prevent UB when computing abs value for num opcode serialize

[philmb3487]

• It seems that the original author of the snippet used glibc code: https://github.com/lattera/glibc/blob/master/stdlib/abs.c

However this is undefined behaviour under certain circumstances: https://stackoverflow.com/questions/17313579/is-there-a-safe-way-to-get-the-unsigned-absolute-value->

A relevant example can be found at : https://godbolt.org/z/yRwtCG

This was first reported in: bitcoin/bitcoin#18046

[santos177]

utACK. It looks clean using static casts, thanks for that!