Fix(rand): UB on different platform

[wjrforcyber]

This idiom combines two 15-bit rand() samples and is only safe when RAND_MAX == 32767 (MSVC/Windows, reference). On glibc and Apple libc, RAND_MAX == 2147483647 (reference), so rand() << 15 overflows signed int (UB) and
the result goes negative sometimes. The negative value is passed to Vec_WecEntry, tripping assert(i >= 0).