qrexec: process vchan data queue (esp MSG_EXIT_CODE) before sending anything

In case of remote process exit even when some messages are still
waiting, vchan connection can be already closed. If we try to send some
data in this case (for example stdout of local process), there will be
an error, which will terminate qrexec-client-vm/qrexec-agent child. So
first check vchan data (where could be MSG_EXIT_CODE queued) , then
local process.

There is still some race condition in this code - remote process could
exit just after we check vchan, but before we send some data. But this
is much less probable and in the worst case we only loose remote process
exit code.

qrexec/qrexec-agent-data.c

@@ -361,26 +361,6 @@ int process_child_io(libvchan_t *data_vchan,
                 handle_vchan_error("wait");
         }
 
-        if (stdout_fd >= 0 && FD_ISSET(stdout_fd, &rdset)) {
-            switch (handle_input(data_vchan, stdout_fd, stdout_msg_type)) {
-                case -1:
-                    handle_vchan_error("send");
-                    break;
-                case 0:
-                    stdout_fd = -1;
-                    break;
-            }
-        }
-        if (stderr_fd >= 0 && FD_ISSET(stderr_fd, &rdset)) {
-            switch (handle_input(data_vchan, stderr_fd, MSG_DATA_STDERR)) {
-                case -1:
-                    handle_vchan_error("send");
-                    break;
-                case 0:
-                    stderr_fd = -1;
-                    break;
-            }
-        }
         /* handle_remote_data will check if any data is available */
         switch (handle_remote_data(data_vchan, stdin_fd, &remote_process_status)) {
             case -1:
@@ -403,6 +383,26 @@ int process_child_io(libvchan_t *data_vchan,
                     return remote_process_status;
                 break;
         }
+        if (stdout_fd >= 0 && FD_ISSET(stdout_fd, &rdset)) {
+            switch (handle_input(data_vchan, stdout_fd, stdout_msg_type)) {
+                case -1:
+                    handle_vchan_error("send");
+                    break;
+                case 0:
+                    stdout_fd = -1;
+                    break;
+            }
+        }
+        if (stderr_fd >= 0 && FD_ISSET(stderr_fd, &rdset)) {
+            switch (handle_input(data_vchan, stderr_fd, MSG_DATA_STDERR)) {
+                case -1:
+                    handle_vchan_error("send");
+                    break;
+                case 0:
+                    stderr_fd = -1;
+                    break;
+            }
+        }
     }
     return child_process_status;
 }