pscp: clear act->buf after receiving 'T' command.

Without this missing line, if you tried to download a file in SCP mode
using the -p option, the payload of the 'T' command (file times) would
still be sitting in act->buf when we went back round the loop, so the
payload of the followup 'C' or 'D' would be appended to it, leading to
a massive misparse and a complaint about illegal file renaming.

pscp.c

@@ -1405,6 +1405,7 @@ int scp_get_sink_action(struct scp_sink_action *act)
 			   &act->mtime, &act->atime) == 2) {
 		    act->settime = true;
                     backend_send(backend, "", 1);
+                    act->buf->len = 0;
 		    continue;	       /* go round again */
 		}
 		bump("Protocol error: Illegal time format");