Checked Size Arithmetic

ISO C. Implement int alloc_safe(size_t count, size_t elemsz, size_t *out_bytes) — compute count*elemsz for allocation; on overflow of the multiplication, or count/elemsz 0 (refuse zero-size asks), return -1 and leave *out_bytes alone; else store the product and return 0. (On this platform you may detect overflow via division: count > SIZE_MAX / elemsz.)