Clean up handling of the return value from sftp_find_request. In many

Clean up handling of the return value from sftp_find_request. In many
places we simply enforce by assertion that it will match the request
we sent out a moment ago: in fact it can also return NULL, so it makes
more sense to report a proper error message if it doesn't return the
expected value, and while we're at it, have that error message
whatever message was helpfully left in fxp_error() by
sftp_find_request when it failed.

To do this, I've written a centralised function in psftp.c called
sftp_wait_for_reply, which is handed a request that's just been sent
out and deals with the mechanics of waiting for its reply, returning
the reply when it arrives, and aborting with a sensible error if
anything else arrives instead. The numerous sites in psftp.c which
called sftp_find_request have all been rewritten to do this instead,
and as a side effect they now look more sensible. The only other uses
of sftp_find_request were in xfer_*load_gotpkt, which had to be
tweaked in its own way.

While I'm here, also fix memory management in sftp_find_request, which
was freeing its input packet on some but not all error return paths.

git-svn-id: svn://svn.tartarus.org/sgt/putty@9894 cda61777-01e9-0310-a592-d414129be87e