SDL_TEXTINPUT event data is NULL terminated

On OS X, I get the following crash on TextInputEvent events (including a trace of the argument to `ccharStringToText`):

    [97,0,0,-96,80,-81,-40,-24,127,127,0,0,56,101,-115,118,-1,127,0,0,80,-81,-40,-24,127,127,0,0,-64,0,74,-122]
    main: Cannot decode byte '\xa0': Data.Text.Internal.Encoding.decodeUtf8: Invalid UTF-8 stream

It looks null terminated, and this patch fixes the crash. I think perhaps a similar change is required in the SDL_TEXTEDITING clause, however I wasn't able to produce one of those events. (I tried using both `SDL.startTextInput rect` and `SDL.Raw.startTextInput` before the event loop).