Fix grab client resume.

Fix grab client resume.

An active grab client is moved to the inactive list if para_server
stops playing and the grab client is not operating in one-shot mode. In
this state, despite its buffer tree node pointer being NULL, the task
associated with the grab client remains active. This causes para_server
to abort due to the assertion btrn != NULL in btr_node_status().

Fix this bug by always unregistering the task, one-shot mode or not,
and re-registering it later, at the same time the new buffer tree
node for the grab client is allocated.