Fixed broken submsg support in upb_streamdata.

core/upb_stream.c

@@ -14,11 +14,14 @@
 void upb_streamdata(upb_src *src, upb_sink *sink, upb_status *status) {
   upb_fielddef *f;
   upb_string *str = NULL;
+  int depth = 0;
+  while(1) {
     while((f = upb_src_getdef(src)) != NULL) {
       CHECKSINK(upb_sink_putdef(sink, f));
       if(upb_issubmsg(f)) {
-      // We always recurse into submessages, but the putdef above already told
+        upb_src_startmsg(src);
-      // the sink that.
+        upb_sink_startmsg(sink);
+        ++depth;
       } else if(upb_isstring(f)) {
         str = upb_string_tryrecycle(str);
         CHECKSRC(upb_src_getstr(src, str));
@@ -32,6 +35,9 @@ void upb_streamdata(upb_src *src, upb_sink *sink, upb_status *status) {
     }
     // If we're not EOF now, the loop terminated due to an error.
     CHECKSRC(upb_src_eof(src));
+    if (depth == 0) break;
+    --depth;
+  }
   return;
 
 src_err: