Add basic protection aginst circular references in pycdas and pycdc.

This fixes the last case of fuzzer errors detected by #572.

pycdas.cpp

@@ -4,6 +4,7 @@
 #include <string>
 #include <iostream>
 #include <fstream>
+#include <unordered_set>
 #include "pyc_module.h"
 #include "pyc_numeric.h"
 #include "bytecode.h"
@@ -73,6 +74,8 @@ static void iprintf(std::ostream& pyc_output, int indent, const char* fmt, ...)
     va_end(varargs);
 }
 
+static std::unordered_set<PycObject *> out_seen;
+
 void output_object(PycRef<PycObject> obj, PycModule* mod, int indent,
                    unsigned flags, std::ostream& pyc_output)
 {
@@ -81,6 +84,12 @@ void output_object(PycRef<PycObject> obj, PycModule* mod, int indent,
         return;
     }
 
+    if (out_seen.find((PycObject *)obj) != out_seen.end()) {
+        fputs("WARNING: Circular reference detected\n", stderr);
+        return;
+    }
+    out_seen.insert((PycObject *)obj);
+
     switch (obj->type()) {
     case PycObject::TYPE_CODE:
     case PycObject::TYPE_CODE2:
@@ -246,6 +255,8 @@ void output_object(PycRef<PycObject> obj, PycModule* mod, int indent,
     default:
         iprintf(pyc_output, indent, "<TYPE: %d>\n", obj->type());
     }
+
+    out_seen.erase((PycObject *)obj);
 }
 
 int main(int argc, char* argv[])