avcodec/cbs_h265: fix writing extension_data bits

We only care about the right most bit.
Signed-off-by: James Almer <jamrial@gmail.com>
(cherry picked from commit 38d1815c)

libavcodec/cbs_h265_syntax_template.c

@@ -80,7 +80,7 @@ static int FUNC(extension_data)(CodedBitstreamContext *ctx, RWContext *rw,
     }
 #else
     for (k = 0; k < current->bit_length; k++)
-        xu(1, extension_data, current->data[k / 8] >> (7 - k % 8), 0, 1);
+        xu(1, extension_data, current->data[k / 8] >> (7 - k % 8) & 1, 0, 1);
 #endif
     return 0;
 }