Revert "Move the saved_wb.bit_buffer += length_field stmt"

This reverts commit c82153045f0c8444f3225874a30c565bc1cead5c.

Reason for revert: The saved_wb.bit_buffer != NULL test cannot be
omitted.

Change-Id: I167866e6a015e33c1cc49652c29da55fa6fbdd2b

av1/encoder/bitstream.c

diff --git a/av1/encoder/bitstream.c b/av1/encoder/bitstream.c
index ad702bd..f13c9d7 100644
--- a/av1/encoder/bitstream.c
+++ b/av1/encoder/bitstream.c
@@ -4133,6 +4133,7 @@
   const int write_frame_header =
       (cpi->num_tg > 1 || encode_show_existing_frame(cm));
   struct aom_write_bit_buffer saved_wb = { NULL, 0 };
+  size_t length_field = 0;
   if (write_frame_header) {
     // Write Frame Header OBU.
     fh_info.frame_header = data;
@@ -4142,11 +4143,7 @@
     obu_payload_size = write_frame_header_obu(cpi, &cpi->td.mb.e_mbd, &saved_wb,
                                               data + obu_header_size, 1);
 
-    size_t length_field =
-        av1_obu_memmove(obu_header_size, obu_payload_size, data);
-    // Since length_field is determined adaptively after frame header
-    // encoding, saved_wb must be adjusted accordingly.
-    saved_wb.bit_buffer += length_field;
+    length_field = av1_obu_memmove(obu_header_size, obu_payload_size, data);
     if (av1_write_uleb_obu_size(obu_header_size, obu_payload_size, data) !=
         AOM_CODEC_OK) {
       return AOM_CODEC_ERROR;
@@ -4160,6 +4157,12 @@
   if (encode_show_existing_frame(cm)) {
     data_size = 0;
   } else {
+    // Since length_field is determined adaptively after frame header
+    // encoding, saved_wb must be adjusted accordingly.
+    if (saved_wb.bit_buffer != NULL) {
+      saved_wb.bit_buffer += length_field;
+    }
+
     //  Each tile group obu will be preceded by 4-byte size of the tile group
     //  obu
     data_size = write_tiles_in_tg_obus(