Keep counted reference to module

+
+    - A counted reference was not kept to the module, so if the entry
+        in sys.modules was replaced, the module was freed prematurely.
+        [Franck Pommerau]

Cython/Compiler/ModuleNode.py

@@ -1543,6 +1543,9 @@ class ModuleNode(Nodes.Node, Nodes.BlockNode):
             "if (!%s) %s;" % (
                 env.module_cname,
                 code.error_goto(self.pos)));
+        code.putln(
+            "Py_INCREF(%s);" %
+                env.module_cname)
         code.putln(
             '%s = PyImport_AddModule(__Pyx_BUILTIN_MODULE_NAME);' %
                 Naming.builtins_cname)