SandboxedExtensionUnpacker::ValidateSignature should check for an empty signature

      Without an expicit check we crash on a hard int3 when trying to allocate 0 bytes.

BUG=62791
TEST=None.
Review URL: http://codereview.chromium.org/4723007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@65821 0039d316-1c4b-4281-b951-d872f2087c98