tracing: Fix use-after-free in hist_register_trigger()

This fixes a use-after-free case flagged by KASAN; make sure the test
happens before the potential free in this case.

Link: http://lkml.kernel.org/r/48fd74ab61bebd7dca9714386bb47d7c5ccd6a7b.1467247517.git.tom.zanussi@linux.intel.com



Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>