decode/udp: fix payload_len calculation

Fix payload_len calculation post removal of the condition that returned
error code if the length to the decode fn did not match the length of
header from the UDP packet.

Bug 5379

(cherry picked from commit f941ceae2b)

src/decode-udp.c

@@ -60,12 +60,16 @@ static int DecodeUDPPacket(ThreadVars *t, Packet *p, const uint8_t *pkt, uint16_
         // packet can still be valid, keeping for consistency with decoder.udp.hlen_invalid event
         ENGINE_SET_INVALID_EVENT(p, UDP_HLEN_INVALID);
     }
+    if (unlikely(UDP_GET_LEN(p) < UDP_HEADER_LEN)) {
+        ENGINE_SET_INVALID_EVENT(p, UDP_LEN_INVALID);
+        return -1;
+    }
 
     SET_UDP_SRC_PORT(p,&p->sp);
     SET_UDP_DST_PORT(p,&p->dp);
 
     p->payload = (uint8_t *)pkt + UDP_HEADER_LEN;
-    p->payload_len = len - UDP_HEADER_LEN;
+    p->payload_len = UDP_GET_LEN(p) - UDP_HEADER_LEN;
 
     p->proto = IPPROTO_UDP;