Raise runtime error on module import failure to avoid segfault

Author: overstre

confluent_kafka/src/confluent_kafka.c

@@ -1959,6 +1959,13 @@ PyObject *cfl_PyObject_lookup (const char *modulename, const char *typename) {
                 return NULL;
         }
 
+        if (!module) {
+                PyErr_Format(PyExc_RuntimeError,
+                             "Module %s import failed.",
+                             modulename);
+                return NULL;
+        }
+
         obj = PyObject_GetAttrString(module, typename);
         if (!obj) {
                 Py_DECREF(module);