MT#61856 control_ng: hardening for `bencode_pretty_print()`

The dict pretty printer advances from key to value
by `chld = chld->sibling` and then prints the `chld`.

If a malformed dict ever reaches this func
with an odd number of child nodes, this can dereference NULL.

Just add guard against NULL.

Change-Id: Ia24671a5eba06dda8c48515fd9dc45fe7a9ec371

daemon/control_ng.c

@@ -629,6 +629,8 @@ static void bencode_pretty_print(bencode_item_t *el, GString *s) {
 				g_string_append(s, sep);
 				bencode_pretty_print(chld, s);
 				g_string_append(s, ": ");
+				if (!chld->sibling)
+					break;
 				chld = chld->sibling;
 				bencode_pretty_print(chld, s);
 				sep = ", ";