perf tools: Do backtrace post unwind only if we regs and stack were captured

perf tools: Do backtrace post unwind only if we regs and stack were captured

Bail out without error if we want to do backtrace post unwind, but were
not able to capture user registers or user stack during the record
phase, which is possible and valid case.

Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1347295819-23177-2-git-send-email-jolsa@redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>