bridge: Cancel prompt if zero bytes received by cockpit-askpass
bridge: Cancel prompt if zero bytes received by cockpit-askpass

If no bytes are received by cockpit-askpass when reading back
an "authorize" message, this means that the prompt has been
cancelled due to the session shutting down or similar.

Closes #6155
Reviewed-by: Peter <a316049dd34607c62d4611d63ab22a6454add223@redhat.com>

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