pack-bitmap: introduce bitmap_writer_free()

Now that there is clearer memory ownership around the bitmap_writer
structure, introduce a bitmap_writer_free() function that callers may
use to free any memory associated with their instance of the
bitmap_writer structure.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

midx-write.c

@@ -853,6 +853,7 @@ static int write_midx_bitmap(const char *midx_name,
 cleanup:
 	free(index);
 	free(bitmap_name);
+	bitmap_writer_free(&writer);
 
 	trace2_region_leave("midx", "write_midx_bitmap", the_repository);