perf report: Don't call map__kmap if map is NULL.

report__warn_kptr_restrict() calls map__kmap(kernel_map) before checking
kernel_map againest NULL.

Which is dangerous, since map__kmap() will return a invalid and not NULL
address.

It will trigger a warning message in map__kmap() after the patch "perf:
kmaps: enforce usage of kmaps to protect futher bugs." was applied.

This patch fixes it by adding the missing checking.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
Link: http://lkml.kernel.org/r/1428490772-135393-1-git-send-email-wangnan0@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-report.c

@@ -347,7 +347,7 @@ static int perf_evlist__tty_browse_hists(struct perf_evlist *evlist,
 static void report__warn_kptr_restrict(const struct report *rep)
 {
 	struct map *kernel_map = rep->session->machines.host.vmlinux_maps[MAP__FUNCTION];
-	struct kmap *kernel_kmap = map__kmap(kernel_map);
+	struct kmap *kernel_kmap = kernel_map ? map__kmap(kernel_map) : NULL;
 
 	if (kernel_map == NULL ||
 	    (kernel_map->dso->hit &&