Fixed compilation errors by adding casts

[jbunton-atlassian]

Trying to run npm install node-stringprep on Arch Linux with gcc 7.1.1 causes the following errors:

../node-stringprep.cc:122:35: error: invalid conversion from ‘uint16_t* {aka short unsigned int*}’ to ‘const UChar* {aka const char16_t*}’ [-fpermissive]
                                   *str, str.length(),
                                   ^~~~

I've fixed them just by adding explicit casts.