Fix: statedump: check task_active_pid_ns return value for NULL

The lttng-statedump checks the return value of task_active_pid_ns()
before each use within trace_lttng_statedump_process_state(), but misses
the NULL check before dereferencing pid_ns->parent.

This race happens if a task exists in "dead" state while the statedump
iterates on that task.

Reported-by: Li Zhou <li.zhou@windriver.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>