gcc warns memcpy will always overflow: memcpy(&icmp, p, sizeof(subtcp)); (Mac OS)

I get this compiler warning on Mac OS.

This looks like a buffer overflow that needs to get fixed.

```
gcc -c -O2 -Wall -g scan.c
scan.c:459:4: warning: 'memcpy' will always overflow; destination buffer has size 8, but size argument is 20 [-Wbuiltin-memcpy-chk-size]
memcpy(&icmp, p, sizeof(subtcp));
^
1 warning generated.
```