perf record: Fix segfault with --no-mmap-pages

perf record: Fix segfault with --no-mmap-pages

Adrian reported a segfault when using --no-out-pages:

$ tools/perf/perf record -vv --no-out-pages uname
Segmentation fault (core dumped)

The same occurs with --no-mmap-pages. Fix by checking that str is
non-NULL before parsing it.

Signed-off-by: David Ahern <dsahern@gmail.com>
Reported-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/1384267617-3446-2-git-send-email-dsahern@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>