Fix bad guard

and add the test that exposed it.

addGuard(obj) is dangerous because obj could be deallocated+reallocated
in the same spot, which is what ended up happening here.

src/runtime/capi.cpp

@@ -1572,6 +1572,7 @@ Box* BoxedCApiFunction::tppCall(Box* _self, CallRewriteArgs* rewrite_args, ArgPa
 
     if (rewrite_args) {
         rewrite_args->obj->addGuard((intptr_t)self);
+        rewrite_args->rewriter->addGCReference(self);
     }
 
     int flags = self->method_def->ml_flags;

test/tests/elementree_test.py

@@ -39,8 +39,18 @@ def test(ET):
         name = country.get('name')
         print name, rank
 
+    it = root.iter
+    i = it()
+    for e in it():
+        assert not isinstance(e, str)
+
 import xml.etree.ElementTree as Python_ET
 import xml.etree.cElementTree as CAPI_ET
 
-test(Python_ET)
-test(CAPI_ET)
+for i in xrange(20):
+    print
+    print i
+    test(Python_ET)
+    test(CAPI_ET)
+    test(CAPI_ET)
+    test(CAPI_ET)