No check for overflow in mpz/mul.c

[Marc Glisse]

Hello,

in mpz_realloc, we check if new_alloc > INT_MAX and produce an error in 
that case.

In mul.c, we use __GMP_ALLOCATE_FUNC_LIMBS directly and store "ALLOC (w) = 
wsize". I think that may explain why we silently get a wrong result in 
https://stackoverflow.com/q/79483605/1918193 . We should probably add a 
check, or see if we can use one of the macros/functions that already do 
the check.

On windows, with a 32-bit long (and thus mp_size_t), it looks like 
usize+vsize could instead overflow and make mpn_mul write in unallocated 
memory, but that seems to be the case throughout the library, and at least 
compiling with -fsanitize=undefined should stop it (whereas it does not 
mind a long->int conversion).

-- 
Marc Glisse