Fix UTF32 error in remove_corrupt_utf8

This fixes the error:

    julia> remove_corrupt_utf8!(sd)
    ERROR: ArgumentError("UTF32String data must be NULL-terminated")
     in UTF32String at /Users/jiahao/local/src/julia/usr/lib/julia/sys.dylib
     in remove_corrupt_utf8 at
    /Users/jiahao/.julia/v0.4/TextAnalysis/src/preprocessing.jl:16
     in remove_corrupt_utf8! at
    /Users/jiahao/.julia/v0.4/TextAnalysis/src/preprocessing.jl:24