Initialize bug_report_ to NULL so as make sure invalid pointer is not freed.

      BUG=67393
TEST=NONE

Review URL: http://codereview.chromium.org/6066002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@69721 0039d316-1c4b-4281-b951-d872f2087c98