xfer_{up,down}load_gotpkt free their input sftp_packet as a side

effect of handling it, but they do not free it if it isn't a packet
they recognise as part of their upload/download. Invent a return value
that specifically signals this, and consistently free pktin at every
call site if that return value comes back. Also, ensure that that
return value also always comes with something meaningful in fxp_error.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9915 cda61777-01e9-0310-a592-d414129be87e