Error in internal buffering in trace output

Imported from http://bugzilla.roxen.com/bugzilla/show_bug.cgi?id=1053

Reported by @marcus

Running the following program in pike7.1.45

---8<---

mapping x = ([]);
class y { int `==(object z) { return 0; } };

void use(mixed _) { }

void main()
{
  x[y()] = 1;
  trace(2);
  x[y()] = 1;
  use(x);
}

---8<---

gives a fatal error:

-   Return: 0
- tracekrash.pike:   9: 8314d74->y()
-   Return: object
- tracekrash.pike:  12: 8314d54->`==(object)
-   Return: 0
/home/marcus/pike/src/dynamic_buffer.c:55: Fatal error:
Error in internal buffering.
Attempting to dump backlog (may fail)...
Backtrace at time of fatal:
tracekrash.pike:12: main()
Abort (core dumped)