Set sock variable to -1 if no persistent conn exists

Wget should set sock variable to -1 if no persistent
connection exists. Function persistent_available_p()
tests persistent connection but if test_socket_open()
fails it closes the socket but will not set sock variable
to -1. After returning from persistent_available_p()
it is possible that sock has still value of already
closed connection.

Signed-off-by: Tomas Hozza <thozza@redhat.com>