Segfaults in malloc_consolidate()

My application segfaults sometimes and mainly in malloc_consolidate() called by format when I look at the backtrace in gdb.

I verified that the machine has enough memory available, it didn't even start swapping. I checked ulimits for data segement and max memory size and both are set to 'unlimited'. I also ran the application under valgrind and didn't find any memory errors.

Now I'm out of ideas what else might be causing these segfaults. Any Ideas ?

code： (https://i.stack.imgur.com/XuNeq.png) gdb: (https://i.stack.imgur.com/pWbD0.png)