perf trace: Handle perf.data files with no tracepoints

Before:

  perf trace -i perf.data
  Segmentation fault (core dumped)
  #

After:

 # perf trace -i perf.data
 Data file does not have raw_syscalls:sys_enter events
 #

When there are no tracepoints in a perf.data file the struct pevent
that contains the list of tracepoints that will be used to lookup the
tracepoint id by name will not be populated, causing a NULL deref.

And we don't need to do all that dance to look at pevents for an entry
with a slighly different name to then lookup the tracepoint by its id on
the evlist, just use the perf_evlist__find_tracepoint_by_name() routine,
that will find the tracepoint, if present.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-egcm21k1e6gcyxpcgjxtmsq3@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>