tracing: checking for NULL instead of IS_ERR()

tracing_map_elt_alloc() returns ERR_PTRs on error, never NULL.

Fixes: 08d43a5f ('tracing: Add lock-free tracing_map')
Link: http://lkml.kernel.org/r/20160423102347.GA11136@mwanda



Acked-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>