Merge pull request #1520 from ctiller/cancel-after-invoke

If we have closed the transport, dont try reading

src/core/transport/chttp2_transport.c

@@ -1927,8 +1927,10 @@ static void recv_data(void *tp, gpr_slice *slices, size_t nslices,
       break;
     case GRPC_ENDPOINT_CB_OK:
       lock(t);
+      if (t->cb) {
         for (i = 0; i < nslices && process_read(t, slices[i]); i++)
           ;
+      }
       unlock(t);
       keep_reading = 1;
       break;