There are some interesting aspects to this. One, the problem affects only Android users of the app. Secondly, WhatsApp does have a character limit of 6000. But at that number, it isn't able to block malicious users from sending an app-crashing message.
This vulnerability is the perfect sneak attack. Because the message that sets off the crashing of the mobile app is also destroyed in the process, there is no evidence that can be used to accuse someone of setting off the crash.
Facebook closed on its acquisition of WhatsApp in October 2014, and the final purchase price came to more than $21 billion. We wouldn't be surprised to see Facebook push out a patch that closes this exploit.
Thanks for the tip!