Reader::parse copies string data with capacity() instead of size()

[baustinanki]

Our valgrind tests, some of which use jsoncpp, detected an uninitialized value being used when creating a std::string of a certain size and passing it into Reader::parse. The std::string, which we create with a string literal, has a size() of 54 but a capacity() of 63. After being passed into Reader::parse, this code runs:

bool
Reader::parse(const std::string& document, Value& root, bool collectComments) {
  JSONCPP_STRING documentCopy(document.data(), document.data() + document.capacity());
  std::swap(documentCopy, document_);

This results in document_.size() being 63; the Reader is now operating on a string that is different than the one we passed in. Our valgrind errors suggest that the extra bytes here are not zero-initialized and are eventually read down the line in Reader::skipSpaces().

I believe the call to capacity() in that code above should instead be a call to size().

[cinemast]

@cdunn2001 can we fix this? It introduces memory troubles for all reverse-dependencies. Can you fix that?

[cdunn2001]

@cinemast, ok. I'll address the comments in #580 and push this weekend.