updown: fix segfault when interface name can't be resolved

updown: fix segfault when interface name can't be resolved

The child_updown() function sets up environment variables to the updown
script. Sometimes call to hydra->kernel_interface->get_interface() could
fail and iface variable could be left uninitialized. This patch fixes
this issue by passing "unknown" as interface name.

Here is the stacktrace:

0  0x00007fa90791f445 in raise () from /lib/x86_64-linux-gnu/libc.so.6
1  0x00007fa907922bab in abort () from /lib/x86_64-linux-gnu/libc.so.6
2  0x0000000000401ed7 in segv_handler (signal=11) at charon.c:183
3  <signal handler called>
4  0x00007fa90793221f in vfprintf () from /lib/x86_64-linux-gnu/libc.so.6
5  0x00007fa9079f0580 in __vsnprintf_chk () from /lib/x86_64-linux-gnu/libc.so.6
6  0x00007fa9079f04c8 in __snprintf_chk () from /lib/x86_64-linux-gnu/libc.so.6
7  0x00007fa8f9b95b86 in snprintf (
    __fmt=0x7fa8f9b961b8 "2>&1 PLUTO_VERSION='1.1' PLUTO_VERB='%s%s%s' PLUTO_CONNECTION='%s' PLUTO_INTERFACE='%s' PLUTO_REQID='%u' PLUTO_ME='%H' PLUTO_MY_ID='%Y' PLUTO_MY_CLIENT='%H/%u' PLUTO_MY_PORT='%u' PLUTO_MY_PROTOCOL='%u"..., __n=1024, __s=0x7fa8f7923440 "2>&1 PLUTO_VERSION='1.1' PLUTO_VERB='up-host' PLUTO_CONNECTION='remote-40.0.0.40' PLUTO_INTERFACE='\367\250\177")
    at /usr/include/x86_64-linux-gnu/bits/stdio2.h:65
8  child_updown (this=0x8486b0, ike_sa=0x7fa8e4005f80, child_sa=0x7fa8d4008290, up=true) at updown_listener.c:308
9  0x00007fa907ecc11c in ?? () from /usr/lib/strongswan/libcharon.so.0
10 0x00007fa907ef89bf in ?? () from /usr/lib/strongswan/libcharon.so.0
11 0x00007fa907ef2fc8 in ?? () from /usr/lib/strongswan/libcharon.so.0
12 0x00007fa907ee84ff in ?? () from /usr/lib/strongswan/libcharon.so.0
13 0x00007fa907ee3067 in ?? () from /usr/lib/strongswan/libcharon.so.0
14 0x00007fa90835e8fb in ?? () from /usr/lib/strongswan/libstrongswan.so.0
15 0x00007fa908360d30 in ?? () from /usr/lib/strongswan/libstrongswan.so.0
16 0x00007fa907cade9a in start_thread () from /lib/x86_64-linux-gnu/libpthread.so.0
17 0x00007fa9079db4bd in clone () from /lib/x86_64-linux-gnu/libc.so.6
18 0x0000000000000000 in ?? ()

Signed-Off-By: Ansis Atteka <aatteka@nicira.com>