Added protection for GetCapabilities() failure.

Added protection for GetCapabilities() failure.

On GetCapabilities() failure, caps.cDestinations is left uninitialized.
Without a protection the following code runs in a random loop
in the worst case up to 0xFFFFFFFF times.
        for (destId = 0; destId < caps.cDestinations; destId++)
        {
            GetDestinationLineInfo(mixId, destId, destLine);

BUG=webrtc:4882
Committed: https://crrev.com/a12ba5502c09b56aa6f8e101e6af7f2d12440663
Cr-Commit-Position: refs/heads/master@{#9663}

[braveyao1, 2015-07-30 06:17:05]

braveyao@webrtc.org changed reviewers:
+ braveyao@webrtc.org, henrika@webrtc.org

[braveyao1, 2015-07-30 06:17:06]

Please have a look when you are available.

[henrika_webrtc, 2015-07-30 12:01:53]

LGTM

[braveyao1, 2015-07-31 03:10:11]

The CQ bit was checked by braveyao@webrtc.org

[commit-bot: I haz the power, 2015-07-31 03:10:25]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1269563002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1269563002/1

[commit-bot: I haz the power, 2015-07-31 03:51:44]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-07-31 03:51:51]

Patchset 1 (id:??) landed as
https://crrev.com/a12ba5502c09b56aa6f8e101e6af7f2d12440663
Cr-Commit-Position: refs/heads/master@{#9663}