perf probe: Prevent segfault when reading probe point with absolute address

'perf probe -l' panic if there is a manually inserted probing point with
absolute address. For example:

  # echo 'p:probe/abs_ffffffff811e6615 0xffffffff811e6615' > /sys/kernel/debug/tracing/kprobe_events
  # perf probe -l
  Segmentation fault (core dumped)

This patch fix this problem by considering the situation that
"tp->symbol == NULL" in find_perf_probe_point_from_dwarf() and
find_perf_probe_point_from_map().

After this patch:

  # perf probe -l
  probe:abs_ffffffff811e6615 (on SyS_write+5@fs/read_write.c)

And when debug info is missing:

  # rm -rf ~/.debug
  # mv /lib/modules/4.2.0-rc1+/build/vmlinux /lib/modules/4.2.0-rc1+/build/vmlinux.bak
  # perf probe -l
  probe:abs_ffffffff811e6615 (on sys_write+5)

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: pi3orama@163.com
Link: http://lkml.kernel.org/r/1440509256-193590-1-git-send-email-wangnan0@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/probe-event.c

@@ -515,7 +515,7 @@ static int find_perf_probe_point_from_dwarf(struct probe_trace_point *tp,
 		if (ret < 0)
 			goto error;
 		addr += stext;
-	} else {
+	} else if (tp->symbol) {
 		addr = kernel_get_symbol_address_by_name(tp->symbol, false);
 		if (addr == 0)
 			goto error;
@@ -1815,17 +1815,17 @@ static int find_perf_probe_point_from_map(struct probe_trace_point *tp,
 {
 	struct symbol *sym = NULL;
 	struct map *map;
-	u64 addr;
+	u64 addr = tp->address;
 	int ret = -ENOENT;
 
 	if (!is_kprobe) {
 		map = dso__new_map(tp->module);
 		if (!map)
 			goto out;
-		addr = tp->address;
 		sym = map__find_symbol(map, addr, NULL);
 	} else {
-		addr = kernel_get_symbol_address_by_name(tp->symbol, true);
+		if (tp->symbol)
+			addr = kernel_get_symbol_address_by_name(tp->symbol, true);
 		if (addr) {
 			addr += tp->offset;
 			sym = __find_kernel_function(addr, &map);