Fix recovering from possible decompression error. Since

Fix recovering from possible decompression error.  Since
insert_from_gap doesn't always move point, we can't use PT as
the position where the partially decompressed data ends, and
should count how may bytes was produced so far.
* decompress.c (struct decompress_unwind_data): Add nbytes member.
(unwind_decompress): Really delete partially uncompressed data.
(Fzlib_decompress_region): Take decompressed data size into account.