sftp_packetlist_flush: zombies must not have responses already
sftp_packetlist_flush: zombies must not have responses already

When flushing the packetlist, we must only add the request as a zombie
if no response has already been received. Otherwise we could wrongly
make it a zombie even though the response was already received and then
we'd get a zombie stuck there "forever"...

libssh2/libssh2,libssh2/libssh2,libssh2/libssh2,libssh2/libssh2