sprof: check pread size and offset for overflow

[Paul Eggert]

On 2025-10-15 18:53, Collin Funk wrote:
> glibc copies intprops.h from Gnulib, which has INT_ADD_WRAPV, but that
> is just __builtin_add_overflow on all compilers glibc supports. CC'ed
> Paul who can correct me if I am wrong.

You're right.

We could change glibc to use C23-style ckd_add, by adapting Gnulib's 
stdckdint.h machinery to glibc. (It uses intprops.h.)