net: conn: Check that TCP pointer is valid
net: conn: Check that TCP pointer is valid

Saw this crash with heavily loaded system in nucleo_f767zi:

<err> os: ***** MPU FAULT *****
<err> os:   Data Access Violation
<err> os:   MMFAR Address: 0x0
<err> os: r0/a1:  0x800f6d30  r1/a2:  0x80005d84  r2/a3:  0x00000006
<err> os: r3/a4:  0x00000000 r12/ip:  0x00000001 r14/lr:  0x60013f69
<err> os:  xpsr:  0x61000000
<err> os: Faulting instruction address (r15/pc): 0x60014304
<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 0x60014304 points to net_conn_input()

   } else if (IS_ENABLED(CONFIG_NET_TCP) && proto == IPPROTO_TCP) {
	src_port = proto_hdr->tcp->src_port;
60014300:	f8d9 3000 	ldr.w	r3, [r9]
60014304:	881a      	ldrh	r2, [r3, #0]

Signed-off-by: Jukka Rissanen <f9f077d1da4c7aa947f9864fa5a28c3588f6a45d@linux.intel.com>

nashif/zephyr,nashif/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,nashif/zephyr