As it turns out `big.Int#Mul` doesn't like it when you reuse input as
output.
```
name old time/op new time/op delta
WinCounts-8 4.56µs ± 1% 3.90µs ± 2% -14.44% (p=0.000 n=9+10)
name old alloc/op new alloc/op delta
WinCounts-8 3.50kB ± 0% 1.15kB ± 0% -67.23% (p=0.000 n=10+10)
name old allocs/op new allocs/op delta
WinCounts-8 45.0 ± 0% 24.0 ± 0% -46.67% (p=0.000 n=10+10)
```
Signed-off-by: Jakub Sztandera <kubuxu@protocol.ai>
