invalid bug for three-byte characters

https://discourse.julialang.org/t/bug-in-isvalid-with-an-overlong-utf-8-encoded-vector-or-string/15290
```jl
julia> isvalid(String, UInt8[0xf0,128,128,128])
false

julia> isvalid(String, UInt8[0xe0,128,128])
true

julia> isvalid(String, UInt8[0xc0,128])
false
```