[Bug sanitizer/63806] #UBSAN ignores signed char possible overflow

[y.gribov at samsung dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63806

--- Comment #2 from Yury Gribov <y.gribov at samsung dot com> ---
I think Marat meant that narrowing cast from int to char can be undefined and
it makes sense to emit some check for it as well.