[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
Merge pull request #2693 from wuhengzhi/xwalk_2928_cid220017

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