decodebin: only try to expose complete groups

When switching to a new chain it might be that this new chain
is not yet ready to be exposed so check it before exposing.

Can happen with mpegts that might delay adding pads or pushing data
until it has found the PMT/PAT/PCR and that may take a while depending
on the stream.

It happened frequently with HLS:
http://vevoplaylist-live.hls.adaptive.level3.net/vevo/ch1/appleman.m3u8

gst/playback/gstdecodebin2.c

@@ -4352,8 +4352,12 @@ retry:
             &drained, &switched);
         GST_ELEMENT_WARNING (dbin, STREAM, FAILED, (NULL),
             ("all streams without buffers"));
-        if (switched)
-          goto retry;
+        if (switched) {
+          if (gst_decode_chain_is_complete (dbin->decode_chain))
+            goto retry;
+          else
+            return FALSE;
+        }
       }
     }