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
SandboxedExtensionUnpacker::SandboxedExtensionUnpacker(
    const FilePath& crx_path,
    const FilePath& temp_path,
    ResourceDispatcherHost* rdh,
    SandboxedExtensionUnpackerClient* client)
    : crx_path_(crx_path), temp_path_(temp_path),
      thread_identifier_(BrowserThread::ID_COUNT),
      rdh_(rdh), client_(client), got_response_(false) {
}

SandboxedExtensionUnpacker::SandboxedExtensionUnpacker(
    const FilePath& crx_path,
    const FilePath& temp_path,
    ResourceDispatcherHost* rdh,
    SandboxedExtensionUnpackerClient* client)
    : crx_path_(crx_path), temp_path_(temp_path),
      thread_identifier_(BrowserThread::ID_COUNT),
      rdh_(rdh), client_(client), got_response_(false) {
}
