altcp: Fix NULL pointer dereference found by fuzzing

Reproducer (in bash):

base64 -d <<< "H4sIAP/9L2QCA+3WoQ2AMBSE4QoCTFHBBJfgSRF4RDfpRmgmYBpGQRBCk4ZiSfk/+fJMK+5dZRVpzSQzSs6oPierDV4y87WxLQLwE42SfNCdDyHJB9/xZwAARPbMJbUq4JJmu4JVT1cAAACfbGIqoqcMzy90eu+aBw2+N28WFgAA" | gunzip | test/fuzz/lwip_fuzz2

Crash log:

../../src/core/altcp_tcp.c:178:13: runtime error: member access within null pointer of type 'struct tcp_pcb'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ../../src/core/altcp_tcp.c:178:13 in
AddressSanitizer:DEADLYSIGNAL
=================================================================
==192415==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000048 (pc 0x557065081703 bp 0x0aae0cb71204 sp 0x7ffd034dabc0 T0)
==192415==The signal is caused by a READ memory access.
==192415==Hint: address points to the zero page.
    #0 0x557065081703 in altcp_tcp_setup_callbacks /.../lwip/test/fuzz/../../src/core/altcp_tcp.c:178:19
    #1 0x55706508206f in altcp_tcp_setup /.../lwip/test/fuzz/../../src/core/altcp_tcp.c:189:3
    #2 0x55706508206f in altcp_tcp_accept /.../lwip/test/fuzz/../../src/core/altcp_tcp.c:84:5
    #3 0x557065095592 in tcp_input /.../lwip/test/fuzz/../../src/core/tcp_in.c:380:9
    #4 0x5570650e752f in ip4_input /.../lwip/test/fuzz/../../src/core/ipv4/ip4.c:743:9
    #5 0x55706513d4de in ethernet_input /.../lwip/test/fuzz/../../src/netif/ethernet.c:186:9
    #6 0x557064fe0959 in input_pkt /.../lwip/test/fuzz/fuzz_common.c:209:9
    #7 0x557064fdeb6a in input_pkts /.../lwip/test/fuzz/fuzz_common.c:257:9
    #8 0x557064fdeb6a in lwip_fuzztest /.../lwip/test/fuzz/fuzz_common.c:669:3
    #9 0x7ff4f578e189 in __libc_start_call_main csu/../sysdeps/nptl/libc_start_call_main.h:58:16
    #10 0x7ff4f578e244 in __libc_start_main csu/../csu/libc-start.c:381:3
    #11 0x557064f20420 in _start (/.../lwip/test/fuzz/lwip_fuzz2+0x81420) (BuildId: 8680a96430d5749c90111fe9c3a3d4f881a5dbcd)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV /.../lwip/test/fuzz/../../src/core/altcp_tcp.c:178:19 in altcp_tcp_setup_callbacks
==192415==ABORTING
Aborted

src/core/altcp_tcp.c

@@ -75,7 +75,7 @@ static err_t
 altcp_tcp_accept(void *arg, struct tcp_pcb *new_tpcb, err_t err)
 {
   struct altcp_pcb *listen_conn = (struct altcp_pcb *)arg;
-  if (listen_conn && listen_conn->accept) {
+  if (new_tpcb && listen_conn && listen_conn->accept) {
     /* create a new altcp_conn to pass to the next 'accept' callback */
     struct altcp_pcb *new_conn = altcp_alloc();
     if (new_conn == NULL) {