Fix ext-tile encoding when not using compressed header

The compressed header size is now sometimes not signalled and the
encoder code hadn't been updated to match.

Note: This mistake caused the failing test tracked in bug 917 to fail,
      but fixing it doesn't fix the test: the decoder now staggers on
      a little further before getting out of sync.

BUG=aomedia:917

Change-Id: Ic0de99ede778844c4dbdd34dba3fba9c1fa2d95f
diff --git a/av1/encoder/bitstream.c b/av1/encoder/bitstream.c
index b71ba8b..3b95523 100644
--- a/av1/encoder/bitstream.c
+++ b/av1/encoder/bitstream.c
@@ -5299,7 +5299,10 @@
     }
     // TODO(jbb): Figure out what to do if compressed_hdr_size > 16 bits.
     assert(compressed_hdr_size <= 0xffff);
-    aom_wb_write_literal(&saved_wb, compressed_hdr_size, 16);
+    // Fill in the compressed header size (but only if we're using one)
+    if (use_compressed_header(cm)) {
+      aom_wb_write_literal(&saved_wb, compressed_hdr_size, 16);
+    }
   } else {
 #endif  // CONFIG_EXT_TILE
     data += data_size;