Fix segfault with ssl errors.
Fix segfault with ssl errors.

When serf_bucket_response_status gets an ssl error it can close the connection
and call CallbackDone(false).  This nulls async_fetch_.

Only try to use async_fetch_ if serf_bucket_response_status returns success.

webscale-networks/mod_pagespeed,webscale-networks/mod_pagespeed,webscale-networks/mod_pagespeed,webscale-networks/mod_pagespeed,webscale-networks/mod_pagespeed,webscale-networks/mod_pagespeed,webscale-networks/mod_pagespeed,webscale-networks/mod_pagespeed