net/udp: Remove wrong check in udp_readahead

- Before IOB offload, srcaddr / src_addr_size / ifindex are written into
  IOB by iob_trycopyin, so io_pktlen > 0 is always true, this check is
  correct at that time. (It won't fail with zero-length UDP datagram.)
- After IOB offload, srcaddr / src_addr_size / ifindex are written into
  offset 0, without increasing io_pktlen. So this check will fail with
  zero-length UDP datagram now.
- We need to support zero-length UDP datagram and this check is
  unnecessary at this point.
  - https://stackoverflow.com/questions/5307031/how-to-detect-receipt-of-a-0-length-udp-datagram
  - https://github.com/apache/nuttx/blob/nuttx-12.1.0/net/udp/udp_callback.c#L214

Signed-off-by: Zhe Weng <wengzhe@xiaomi.com>

net/udp/udp_recvfrom.c

@@ -170,8 +170,6 @@ static inline void udp_readahead(struct udp_recvfrom_s *pstate)
       FAR void *srcaddr;
       uint8_t ifindex;
 
-      DEBUGASSERT(iob->io_pktlen > 0);
-
       /* Unflatten saved connection information */
 
 #ifdef CONFIG_NETDEV_IFINDEX