fix compilation error

src/core/ext/transport/cronet/transport/cronet_transport.c

@@ -780,8 +780,8 @@ static enum e_op_result execute_stream_op(grpc_exec_ctx *exec_ctx,
     s->cbs = cronet_bidirectional_stream_create(s->curr_ct.engine, s->curr_gs,
                                                 &cronet_callbacks);
     CRONET_LOG(GPR_DEBUG, "%p = cronet_bidirectional_stream_create()", s->cbs);
-    char *url;
+    char *url = NULL;
-    const char *method;
+    const char *method = NULL;
     s->header_array.headers = NULL;
     convert_metadata_to_cronet_headers(
         stream_op->send_initial_metadata->list.head, s->curr_ct.host, &url,