Partly. They were using their own allocator (openssl_malloc()), but even then they would've been OK if it weren't for the OBO error elsewhere in the heartbeat implementation. If they were using an OS-supplied malloc() instead of openssl_malloc(), the bug would've still been exploitable on some operating systems, but not others.

Either way, "don't write your own allocator" is a good lesson to learn.

Unless, of course, you're doing it for fun. In which case, efficient heap management really is a neat exercise.

Heartbleed was due to not checking the bounds.

https://www.openssl.org/news/secadv_20140407.txt

https://xkcd.com/1354/