Fix a typo in ec_byte_write_done().

Introduced by 30df6cf3.
This should have only affected the output in the case where the last
 few extra bits caused us to bust, and wouldn't have prevented us
 from detecting the error.

libcelt/entenc.c

@@ -76,7 +76,7 @@ int ec_byte_write_done(ec_byte_buffer *_b,int _start_bits_available,
          would corrupt the range coder data, and that's more important.*/
       if(_start_bits_available<_end_bits_used){
         _end_bits_used=_start_bits_available;
-        _end_byte&=(1<_start_bits_available)-1;
+        _end_byte&=(1<<_start_bits_available)-1;
         ret=-1;
       }
     }