[Runtime] Add the check for return value of GetDEREncoded().

This patch is to add the check for return value of GetDEREncoded() in
xwalk_contents_client_bridge.cc.

CID=220017

Related to XWALK-2928
(cherry picked from commit 9767ffd50567a82dc231136cb57579a9775162a4)
Merge pull request #2741 from wuhengzhi/crosswalk-10

[Runtime] Add the check for return value of GetDEREncoded().