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.

[originally from svn r9894]

sftp.c

@@ -366,7 +366,6 @@ struct sftp_request *sftp_find_request(struct sftp_packet *pktin)
 
     if (!req || !req->registered) {
 	fxp_internal_error("request ID mismatch\n");
-        sftp_pkt_free(pktin);
 	return NULL;
     }
 
@@ -1203,6 +1202,8 @@ int xfer_download_gotpkt(struct fxp_xfer *xfer, struct sftp_packet *pktin)
     struct req *rr;
 
     rreq = sftp_find_request(pktin);
+    if (!rreq)
+        return 0;            /* this packet doesn't even make sense */
     rr = (struct req *)fxp_get_userdata(rreq);
     if (!rr)
 	return 0;		       /* this packet isn't ours */
@@ -1383,6 +1384,8 @@ int xfer_upload_gotpkt(struct fxp_xfer *xfer, struct sftp_packet *pktin)
     int ret;
 
     rreq = sftp_find_request(pktin);
+    if (!rreq)
+        return 0;            /* this packet doesn't even make sense */
     rr = (struct req *)fxp_get_userdata(rreq);
     if (!rr)
 	return 0;		       /* this packet isn't ours */