widen r * p to u64 in scrypt params bounds check

[dxbjavid]

Noticed the final guard in Params::new computes r * p in u32, but both come from a parsed PHC hash string and the earlier checked_mul guards only bound the usize products. On 64-bit, r=65536 p=65536 reaches this line: the u32 multiply wraps to 0, so r * p >= 0x4000_0000 is false and scrypt accepts params it must reject (debug builds panic on the overflow instead). Widen both operands to u64 before the compare.