tools lib traceevent: Add sanity check to is_timestamp_in_us()

tools lib traceevent: Add sanity check to is_timestamp_in_us()

This patch adds a sanity check to is_timestamp_in_us() input parameter
trace_clock. It avoids a potential segfault in this function for the
case trace_clock is NULL.

Reported-by: Slavomir Kaslev <kaslevs@vmware.com>
Signed-off-by: Tzvetomir Stoyanov <tstoyanov@vmware.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/20181128145552.68c4f87b@gandalf.local.home
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>