pimd: When receiving a packet be more careful with length in pim_pim_…

…packet

a) If the length passed is the header length then it is possible that
assignment of data will happen without data actually existing.

b) Just move the assignment to after we ensure that the pim packet
received is the minimum possible length that can be received.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>

pimd/pim_pim.c

@@ -155,7 +155,7 @@ int pim_pim_packet(struct interface *ifp, uint8_t *buf, size_t len,
 	bool   no_fwd;
 
 #if PIM_IPV == 4
-	if (len < sizeof(*ip_hdr)) {
+	if (len <= sizeof(*ip_hdr)) {
 		if (PIM_DEBUG_PIM_PACKETS)
 			zlog_debug(
 				"PIM packet size=%zu shorter than minimum=%zu",
@@ -189,14 +189,14 @@ int pim_pim_packet(struct interface *ifp, uint8_t *buf, size_t len,
 	iovp->iov_len = pim_msg_len;
 	iovp++;
 
-	header = (struct pim_msg_header *)pim_msg;
 	if (pim_msg_len < PIM_PIM_MIN_LEN) {
 		if (PIM_DEBUG_PIM_PACKETS)
 			zlog_debug(
 				"PIM message size=%d shorter than minimum=%d",
 				pim_msg_len, PIM_PIM_MIN_LEN);
 		return -1;
 	}
+	header = (struct pim_msg_header *)pim_msg;
 
 	if (header->ver != PIM_PROTO_VERSION) {
 		if (PIM_DEBUG_PIM_PACKETS)