Fix: statedump: consistently check task_cred_xxx() return value for NULL

trace_lttng_statedump_process_user_ns() internally checks whether
user_ns is NULL. While this does not appear to be a possible return
value for task_cred_xxx(), err on the safe side and check for NULL here
as well to be consistent with the paranoid behavior of
trace_lttng_statedump_process_user_ns().

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>