Fix a bug where we would read too much data in HTTP bodies or requests. We were using evbuffer_add_buffer, which moved the entire buffer contents. But if we had a valid content_length, we only wanted to move up to the amount of data remaining in ntoread. Our bug would make us put our ntoread in the negative, which would in turn make us read all data until the connection closed. Found by Denis Bilenko. Should fix bug 2963172.