The __cxa_demangle() call was leaking the demangled name buffer
(Confirmed by valgrind).

If I understood the __cxa_demangle() docs right, the issue was that you were
mixing the two operating modes (User buffer vs __cxa_demangle()
allocated buffer).
You passed a garbage pointer to __cxa_demangle(), which expects null
or a pointer to a malloc() allocated block of *size* bytes. Note also size is
0 in your implementation. Basically the leak comes from __cxa_demangle() doing a
realloc() on your garbage pointer.

The fix is as simple as telling __cxa_demangle() to allocate the buffer
itself.

Just for reference, here is the __cxa_demangle() documentation
explaining this behavior:

https://gcc.gnu.org/onlinedocs/libstdc++/libstdc++-html-USERS-4.3/a01696.html

When the stack trace is used directly (and not by a signal),
one may want to hide some of the first stack items because
they will always be the same calls.