[issue1621] Do not assume signed integer overflow behavior

[Gregory P. Smith]

Gregory P. Smith <greg at krypto.org> added the comment:

"""I assume these changes were made to silence warnings from
-Wstrict-overflow, but I don't think that should be a goal:
I'd suggest only making changes where there's a genuine
possibility of overflow (even if it's a remote one), and
leaving the code unchanged if it's reasonably easy to
see that overflow is impossible."""

There is a lot of value in being able to compile with -Wstrict-overflow 
and know that every warning omitted is something to be looked at.  I 
think it is advantageous to make all code pass this.  Having any 
"expected" warnings during compilation tends to lead people to ignore 
all warnings.

That said, I agree those particular examples of unnecessary casts are 
ugly and should be written differently if they are actually done to 
prevent a warning.

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