Fix slavelastic Crash
Fix slavelastic Crash

Fix a simple crash in slavelastic when it fails to retrieve any exit codes from swarm.

BUG=
TEST=


Review URL: http://codereview.chromium.org/10272026

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@134716 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build