fix for importlib.reload

Author: lukasstadler

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/objects/cext/capi/CExtNodes.java

@@ -1977,7 +1977,9 @@ static int doGeneric(CApiContext capiContext, PythonModule module, Object module
                      * similar. We ignore that for now since the size will usually be very small
                      * and/or we could also use a Truffle buffer object.
                      */
-                    module.setNativeModuleState(alloc.alloc(mSize));
+                    Object mdState = alloc.alloc(mSize == 0 ? 1 : mSize); // ensure non-null value
+                    assert mdState != null && !InteropLibrary.getUncached().isNull(mdState);
+                    module.setNativeModuleState(mdState);
                 }
 
                 // parse slot definitions