The sensitive data that was saved in that address is still there. Memory has been freed so the os can use is again but the actual data is still there is memory untill get get overwritten by something else...

The program will work with no problems, but sensitive data that has been used then freed is available for retrieval when bugs like heartbleed are found.

As the article suggests the right way is to clean the data from memory ( by overwriting it with something else) before freeing it.