OpenSSL, like many other software indeed used a custom allocator, but this hasn't much to do with this to anything at all, as the system allocator also strongly favors giving back memory that once belonged to the same process, as it has to zero memory that belonged to other processes first.

This is of course a kernel feature when the lower level primitives are used that ask for blocks of memory from the kernel, which zeroes them if they had belonged to another process prior, and does not when they had not, and thus strongly favors giving back own memory. — allocators, such as the standard library's one or any custom ones, are built on top of these primitives.