CVE-2017-12993/Juniper: Add more bounds checks.

This fixes a buffer over-read discovered by Kamil Frankowicz.

Add tests using the capture files supplied by the reporter(s).
ip_heuristic_guess(netdissect_options *ndo,
                   register const u_char *p, u_int length)
{
    switch(p[0]) {
    case 0x45:
    case 0x46:
    case 0x47:
    case 0x48:
    case 0x49:
    case 0x4a:
    case 0x4b:
    case 0x4c:
    case 0x4d:
    case 0x4e:
    case 0x4f:
	    ip_print(ndo, p, length);
	    break;
    case 0x60:
    case 0x61:
    case 0x62:
    case 0x63:
    case 0x64:
    case 0x65:
    case 0x66:
    case 0x67:
    case 0x68:
    case 0x69:
    case 0x6a:
    case 0x6b:
    case 0x6c:
    case 0x6d:
    case 0x6e:
    case 0x6f:
        ip6_print(ndo, p, length);
        break;
    default:
        return 0; /* did not find a ip header */
        break;
    }
    return 1; /* we printed an v4/v6 packet */
}

ip_heuristic_guess(netdissect_options *ndo,
                   register const u_char *p, u_int length)
{
    switch(p[0]) {
    case 0x45:
    case 0x46:
    case 0x47:
    case 0x48:
    case 0x49:
    case 0x4a:
    case 0x4b:
    case 0x4c:
    case 0x4d:
    case 0x4e:
    case 0x4f:
	    ip_print(ndo, p, length);
	    break;
    case 0x60:
    case 0x61:
    case 0x62:
    case 0x63:
    case 0x64:
    case 0x65:
    case 0x66:
    case 0x67:
    case 0x68:
    case 0x69:
    case 0x6a:
    case 0x6b:
    case 0x6c:
    case 0x6d:
    case 0x6e:
    case 0x6f:
        ip6_print(ndo, p, length);
        break;
    default:
        return 0; /* did not find a ip header */
        break;
    }
    return 1; /* we printed an v4/v6 packet */
}
