pack-bitmap-write.c: avoid uninitialized 'write_as' field

pack-bitmap-write.c: avoid uninitialized 'write_as' field

Prepare to free() memory associated with bitmapped_commit structs by
zero'ing the 'write_as' field.

In ideal cases, it is fine to do something like:

    for (i = 0; i < writer->selected_nr; i++) {
        struct bitmapped_commit *bc = &writer->selected[i];
        if (bc->write_as != bc->bitmap)
            ewah_free(bc->write_as);
        ewah_free(bc->bitmap);
    }

but if not all of the 'write_as' fields were populated (e.g., because
the packing_data given does not form a reachability closure), then we
may attempt to free uninitialized memory.

Guard against this by preemptively zero'ing this field just in case.

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