sendme windows don't stay synced between sides

#6252 (moved) showed us that circwindows grow beyond their initial windows in normal operation. We can't implement the 6252 patch until we fix this bug.

Our friendly irc person suggests

--- relay.orig.c
+++ relay.c
@@ -1263,7 +1263,7 @@
                "'connected' received, no conn attached anymore. Ignoring.");
       return 0;
     case RELAY_COMMAND_SENDME:
-      if (!conn) {
+      if (!rh.stream_id) {
         if (layer_hint) {
           if (layer_hint->package_window + CIRCWINDOW_INCREMENT >
                 CIRCWINDOW_START_MAX) {
@@ -1292,6 +1292,11 @@
         }
         return 0;
       }
+      if (!conn) {
+        log_info(domain,"sendme cell dropped, unknown stream (streamid %d).",
+                 rh.stream_id);
+        return 0;
+      }
       conn->package_window += STREAMWINDOW_INCREMENT;
       log_debug(domain,"stream-level sendme, packagewindow now %d.",
                 conn->package_window);