The proper thing to do here is to add

-Werror=strict-overflow

to the CFLAGS (*before* -Wall -- we should fix the position of -Wall!);
this will turn all those spots into errors, forcing us to fix them, and
alerting users who might be using a newer compiler than we tested with.

This should be done in favor of -fwrapv, but only if strict-overflow is
supported (which we can find out in the same way as we found out whether
-fwrapv is supported).  I think in practice this means GCC 4.2 or newer.

Can someone come up with a patch?