perf report: Fix crash showing warning related to kernel maps

perf report: Fix crash showing warning related to kernel maps

While testing https://lkml.org/lkml/2012/4/10/123 I hit this crash:

(gdb) bt
 0  0x000000000042000f in __cmd_report (rep=0x7fff80cec580) at builtin-report.c:380
 1  cmd_report (argc=0, argv=<optimized out>, prefix=<optimized out>) at builtin-report.c:759
 2  0x0000000000414513 in run_builtin (p=0x7724a8, argc=3, argv=0x7fff80ceca70) at perf.c:273
 3  0x0000000000413d41 in handle_internal_command (argv=0x7fff80ceca70, argc=3) at perf.c:345
 4  run_argv (argv=0x7fff80cec880, argcp=0x7fff80cec88c) at perf.c:389
 5  main (argc=3, argv=0x7fff80ceca70) at perf.c:487

kernel_map can be NULL, so need to handle it while dumping a warning
to user.

v2:
- fixed RB_EMPTY_ROOT check -- desc takes the altnerative output when RB_EMPTY_ROOT is false.

Signed-off-by: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Namhyung Kim <namhyung.kim@lge.com>
Link: http://lkml.kernel.org/r/1334544855-55021-1-git-send-email-dsahern@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>