Fix typos

Throw exception on null characters

Any further data passed via exml_stream:parse/2 to the parser won't be processed as well because rapidxml works on null-terminated strings and treats the whole input as empty. The similar issue happens when null character is in the middle of input (other exception is thrown but it has eof flag as well).

I implemented the fix that checks (in advance) whether the buffer contains null character. If so, it throws the error immediately. This way we can make sure that input received from Erlang will be valid in term of processing.