Fixed overzealous assert().

Makefile

@@ -397,7 +397,7 @@ lang_ext/lua/upb.so: lang_ext/lua/upb.c $(LIBUPB_PIC)
 
 # Python extension #############################################################
 
-PYTHON=python2.6-dbg
+PYTHON=python
 PYTHONEXT=lang_ext/python/build/install/lib/python/upb/__init__.so
 python: $(PYTHONEXT)
 $(PYTHONEXT): $(LIBUPB_PIC) lang_ext/python/upb.c
@@ -405,4 +405,4 @@ $(PYTHONEXT): $(LIBUPB_PIC) lang_ext/python/upb.c
 	$(Q) cd lang_ext/python && $(PYTHON) setup.py build --debug install --home=build/install
 
 pythontest: $(PYTHONEXT)
-	cd lang_ext/python && cp test.py build/install/lib/python && valgrind $(PYTHON) ./build/install/lib/python/test.py
+	cd lang_ext/python && cp test.py build/install/lib/python && $(PYTHON) ./build/install/lib/python/test.py

upb/def.c

@@ -228,8 +228,10 @@ static void upb_fielddef_free(upb_fielddef *f) {
   }
   if (f->def) {
     // We own a ref on the subdef iff we are not part of a msgdef.
-    assert((f->msgdef == NULL) == (upb_dyncast_unresolveddef(f->def) != NULL));
-    if (f->msgdef == NULL) upb_def_unref(f->def);
+    if (f->msgdef == NULL) {
+      if (f->def) upb_downcast_unresolveddef(f->def);  // assert() check.
+      upb_def_unref(f->def);
+    }
   }
   free(f->name);
   free(f);