SF bug #483469: crash on unbounded recursion in __del__.

PyEval_EvalCodeEx():  increment tstate->recursion_depth around the
decref of the frame, because the C stack for this call is still in
use and the decref can lead to __del__ methods getting called.

While this gives tstate->recursion_depth a value proportional to the
depth of the C stack (instead of a small constant no matter how
deeply __del__s recurse), it's not enough to stop the reported crash
when using the default recursion limit on Windows.

Bugfix candidate.

Python/ceval.c

@@ -2560,7 +2560,15 @@ PyEval_EvalCodeEx(PyCodeObject *co, PyObject *globals, PyObject *locals,
 
   fail: /* Jump here from prelude on failure */
 
+	/* decref'ing the frame can cause __del__ methods to get invoked,
+	   which can call back into Python.  While we're done with the
+	   current Python frame (f), the associated C stack is still in use,
+	   so recursion_depth must be boosted for the duration.
+	*/
+	assert(tstate != NULL);
+	++tstate->recursion_depth;
         Py_DECREF(f);
+	--tstate->recursion_depth;
 	return retval;
 }