Synthesize fault_addr on amd64 if it is zero.

[zachriggle]

Fixes Gallopsled/pwntool#1018

[zachriggle]

Yep, works fine.

[*] '/home/pwntools/ctf-solutions/csaw/2017/quals/pilot/core.1467'
    Arch:      amd64-64-little
    RIP:       0x400b35
    RSP:       0x7ffc747f3968
    Exe:       '/home/pwntools/ctf-solutions/csaw/2017/quals/pilot/pilot' (0x400000)
    Fault:     0x6161616c6161616b
[!] cyclic_find() expects 4-byte subsequences by default, you gave 'kaaalaaa'
    Unless you specified cyclic(..., n=8), you probably just want the first 4 bytes.
    Truncating the data at 4 bytes.  Specify cyclic_find(..., n=8) to override this.
[*] Offset: 0x28

[bennofs]

This is not a good test if I understand the issue correctly. From my tests, it seems that fault_addr was only null before this patch if it would've been >= 2**47. This is because on 64 bit, that's where kernel memory is mapped so we get "protection fault" instead of "segfault". A protection fault does not have the same siginfo as a normal segfault: si_code will be SI_KERNEL = 0x80 and there won't be a fault_addr (see http://elixir.free-electrons.com/linux/v4.14-rc8/source/kernel/signal.c#L1052)

[zachriggle]

Fixed in #1031

[zachriggle]

Err Fixed in #1066