This CL initialize NSData in HTTPMultipartUpload.m to nil.

Problem introduced by issue 2764002.

R=qsr@chromium.org

Review URL: https://breakpad.appspot.com/2794002

Patch from Oliver Robin <olivierrobin@chromium.org>.

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1374 4c0a9323-5329-0410-9bdc-e9ce6186880e