Sanitize saltCharacter()

 - the array of salt chars should be const, it's read-only
 - the array of salt chars should be static, since it is
   initialized to a constant value anyway and can live beyond
   this one function
 - sizeof(char) is 1 by the standard, so remove the weird
   array-size calculation: here saltCharacters has type
   (const) char[63], and its sizeof() is 63