* buffer.h (BUF_COMPACT): New macro to follow the common style.

* buffer.h (BUF_COMPACT): New macro to follow the common style.
* buffer.c (Fget_buffer_create): Use it to set compact field of
struct buffer_text to avoid accessing an uninitialized value
when compact_buffer is called for the first time.
(compact_buffer): Use convenient BUF_COMPACT and BUF_MODIFF.