Throw exception on null characters

[kamilwaz]

exml_stream doesn't validate null characters correctly. Here is an example:

> {ok, Parser} = exml_stream:new_parser(),
> exml_stream:parse(Parser, <<"\0<xml>">>).
{ok,{parser,#Ref<0.816531939.1357774849.238018>,
            [<<0,60,120,109,108,62>>]},
    []}

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.

[NelsonVides]

Fantastic, thanks for finding out this bug and fixing! I only have a comment on the algorithm performance 👇🏽

[NelsonVides]

While it is correct, the problem is that std:strlen is an O(n) operation. Decoding xml is already O(n) so this would mean we're scanning the string twice. I'd prefer here introduce the check on the scan that is already happening, I introduced something similar here: #62 (I'm wondering if this bug now is somehow related to those last changes of mine 🤔 )

[NelsonVides]

All right, this seems more optimal, specially that in the most common case result.rest will be empty, so std::strlen will be a noop, perfect 👌🏽