runtime: zero unused portion of temp buffer in stringtoslice{byte,rune}

[randall77]

Maybe we should zero the unused portion of the temp buffer instead of reducing the capacity of the returned slice.

See issue #14232

[randall77]

Zeroing the tmpBuf is only 4 instructions (on amd64), so it is probably worth it. It allows things like the append in the following function to work without allocation:

func g(s, t string) string {
    b := ([]byte)(s)
    b = append(b, ([]byte)(t)...)
    return string(b)
}

[gopherbot]

CL https://golang.org/cl/22424 mentions this issue.