net: ethernet: Check that header is valid

Saw this crash with heavily loaded system in mimxrt1050_evk:

<err> os: ***** MPU FAULT *****
<err> os:   Data Access Violation
<err> os:   MMFAR Address: 0xc
<err> os: r0/a1:  0x80000ab0  r1/a2:  0x800f6a60  r2/a3:  0x00000000
<err> os: r3/a4:  0x800f72a0 r12/ip:  0x00000000 r14/lr:  0x6000eb43
<err> os:  xpsr:  0x41000000
<err> os: Faulting instruction address (r15/pc): 0x6000dc82
<err> os: >>> ZEPHYR FATAL ERROR 0: CPU exception on CPU 0
<err> os: Current thread: 0x80001a18 (rx_workq)
<err> os: Halting system

Where the fault at 0x6000dc82 points to ethernet_recv()

	uint16_t type = ntohs(hdr->type);
6000dc82:	89ab      	ldrh	r3, [r5, #12]

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>

Author: jukkar

subsys/net/l2/ethernet/ethernet.c

@@ -174,10 +174,16 @@ static enum net_verdict ethernet_recv(struct net_if *iface,
 	struct ethernet_context *ctx = net_if_l2_data(iface);
 	struct net_eth_hdr *hdr = NET_ETH_HDR(pkt);
 	uint8_t hdr_len = sizeof(struct net_eth_hdr);
-	uint16_t type = ntohs(hdr->type);
+	uint16_t type;
 	struct net_linkaddr *lladdr;
 	sa_family_t family;
 
+	if (hdr == NULL) {
+		goto drop;
+	}
+
+	type = ntohs(hdr->type);
+
 	if (net_eth_is_vlan_enabled(ctx, iface) &&
 	    type == NET_ETH_PTYPE_VLAN &&
 	    !eth_is_vlan_tag_stripped(iface)) {