change default hash value for FxHasher32

[httaotao]

hello, we find out a bug , there has som code in lib.rs

impl Default for FxHasher32 {
    #[inline]
    fn default() -> FxHasher32 {
        FxHasher32 { hash: 0 }
    }
}

When we set zero into FxHasher32, the value is also zero. However, the normal value should be not zero.

then when we change the default value

impl Default for FxHasher32 {
    #[inline]
    fn default() -> FxHasher32 {
        FxHasher32 { hash: 41 }
    }
}

it's ok.

[hkBst]

The core of the algorithm is:

*hash = hash.rotate_left(ROTATE).bitxor(input).wrapping_mul(MUL);

so if hash starts out as 0 it will quickly become non-zero. So if the zero is problematic for you you can simply hash some constant before you hash your actual data.