tools lib traceevent: test correct variable after allocation

we've tested the wrong variable for allocation failure, fix it to
test the right one.

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/1356120062-2648-1-git-send-email-sasha.levin@oracle.com


Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>