perf tools: Fix parse_events_error dereferences

Parse errors can be reported in struct parse_events_error but the
pointer passed is optional and can be NULL.  Ensure it is not NULL
before dereferencing it.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@gmail.com>
Link: http://lkml.kernel.org/r/1432040746-1755-4-git-send-email-adrian.hunter@intel.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>