pref_counter: tools: report: Robustify in case of weird events

This error condition:

  aldebaran:~/linux/linux/Documentation/perf_counter> perf report
  dso__load_sym: cannot get elf header.
  failed to open: /etc/ld.so.cache
  problem processing PERF_EVENT_MMAP, bailing out

caused the profile to be very short - as the error was at the beginning
of the file and we bailed out completely.

Be more permissive and consider the event broken instead.

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: John Kacur <jkacur@redhat.com>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>