Fix a bug when we can't get default address

When running this code in chromium on a machine with IPv6 disabled, the RTC_DCHECK fails and in release build, it could leak to further crash in chromium's rtc_peer_connection_hanlder.cc.

Here is the right fix.

BUG=webrtc:5061
R=pthatcher@google.com
TBR=pthatcher@webrtc.org

Committed: https://crrev.com/e03cab94c1bac43f4d6c4775023a957f98ee8132
Cr-Commit-Position: refs/heads/master@{#10607}

[guoweis_webrtc, 2015-11-11 18:56:39]

Description was changed from

==========
fix

BUG=
==========

to

==========
When running this code in chromium on a machine with IPv6 disabled, the
RTC_DCHECK fails and in release build, it could leak to further crash in
chromium's rtc_peer_connection_hanlder.cc.

Here is the right fix.

BUG=webrtc:5061
==========

[guoweis_webrtc, 2015-11-11 18:56:39]

guoweis@webrtc.org changed reviewers:
+ pthatcher@webrtc.org

[guoweis_webrtc, 2015-11-11 18:56:54]

Description was changed from

==========
When running this code in chromium on a machine with IPv6 disabled, the
RTC_DCHECK fails and in release build, it could leak to further crash in
chromium's rtc_peer_connection_hanlder.cc.

Here is the right fix.

BUG=webrtc:5061
==========

to

==========
When running this code in chromium on a machine with IPv6 disabled, the
RTC_DCHECK fails and in release build, it could leak to further crash in
chromium's rtc_peer_connection_hanlder.cc.

Here is the right fix.

BUG=webrtc:5061
TBR=pthatcher@webrtc.org
==========

[guoweis_webrtc, 2015-11-11 19:00:36]

Patchset #1 (id:1) has been deleted

[pthatcher, 2015-11-11 19:05:28]

pthatcher@google.com changed reviewers:
+ pthatcher@google.com

[pthatcher, 2015-11-11 19:05:28]

lgtm

It would be good to have a unit test for this.  Can you add one in a follow up
CL?

[guoweis_webrtc, 2015-11-11 19:11:32]

Committed patchset #5 (id:90001) manually as
e03cab94c1bac43f4d6c4775023a957f98ee8132 (presubmit successful).

[commit-bot: I haz the power, 2015-11-11 19:11:36]

Patchset 5 (id:??) landed as
https://crrev.com/e03cab94c1bac43f4d6c4775023a957f98ee8132
Cr-Commit-Position: refs/heads/master@{#10607}