varlink: do not parse invalid messages twice

Upon reception of a message which fails in json_parse(), we would proceed to
parse it again from a deferred callback and hang. Once we have realized that
the message is invalid, let's move the pointer in the buffer even if the
message is invalid. We don't want to look at this data again.

(before) $ build-rawhide/userdbctl --output=json user test.user
n/a: varlink: setting state idle-client
/run/systemd/userdb/io.systemd.Multiplexer: Sending message: {"method":"io.systemd.UserDatabase.GetUserRecord","parameters":{"userName":"test.user","service":"io.systemd.Multiplexer"}}
/run/systemd/userdb/io.systemd.Multiplexer: varlink: changing state idle-client → awaiting-reply
/run/systemd/userdb/io.systemd.Multiplexer: New incoming message: {...}
/run/systemd/userdb/io.systemd.Multiplexer: varlink: changing state awaiting-reply → pending-disconnect
/run/systemd/userdb/io.systemd.Multiplexer: New incoming message: {...}
/run/systemd/userdb/io.systemd.Multiplexer: varlink: changing state pending-disconnect → disconnected
^C

(after) $ n/a: varlink: setting state idle-client
/run/systemd/userdb/io.systemd.Multiplexer: Sending message: {"method":"io.systemd.UserDatabase.GetUserRecord","parameters":{"userName":"test.user","service":"io.systemd.Multiplexer"}}
/run/systemd/userdb/io.systemd.Multiplexer: varlink: changing state idle-client → awaiting-reply
/run/systemd/userdb/io.systemd.Multiplexer: New incoming message: {...}
/run/systemd/userdb/io.systemd.Multiplexer: Failed to parse JSON: Invalid argument
/run/systemd/userdb/io.systemd.Multiplexer: varlink: changing state awaiting-reply → pending-disconnect
/run/systemd/userdb/io.systemd.Multiplexer: varlink: changing state pending-disconnect → processing-disconnect
Got lookup error: io.systemd.Disconnected
/run/systemd/userdb/io.systemd.Multiplexer: varlink: changing state processing-disconnect → disconnected
Failed to find user test.user: Input/output error

This should fix #16683 and https://bugs.gentoo.org/735072.

(cherry picked from commit 77472d06)