ustream-ssl: skip writing pending data if .eof is true after connect

ustream-ssl: skip writing pending data if .eof is true after connect

Check the .eof member of the underlying ustream after the call to
__ustream_ssl_connect() since existing users of the library appear
to set the eof flag as a way to signal connection termination upon
failing certificate verification.

This is a stop-gap measure to address TALOS-2019-0893 but a proper
API redesign is required to give applications proper control over
whether certificate failures are to be ignored or not and the default
implementation without custom callbacks should always terminate on
verification failures.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>