Fix for verifier failure in wakeuptime

On my system, the manually unrolled stack walker fails the verifier, due
to what seems to me like an incorrect return statement. Setting bp = 0
has no effect, seems that the intent was to return 0 instead.

Not sure why others haven't hit this, but the fix contained here seems
reasonable.
Signed-off-by: Brenden Blanco <bblanco@gmail.com>

tools/wakeuptime.py

@@ -81,7 +81,7 @@ static u64 get_frame(u64 *bp) {
         if (bpf_probe_read(&ret, sizeof(ret), (void *)(*bp+8)))
             return 0;
         if (bpf_probe_read(bp, sizeof(*bp), (void *)*bp))
-            bp = 0;
+            return 0;
         if (ret < __START_KERNEL_map)
             return 0;
         return ret;