page_zip_compress(): Add assertions !c_stream.avail_in where appropriate.
Add assertions on c_stream.next_in.  Correct malformed assertion on
externs > c_stream.next_in.  Note that there may be trailing garbage after
the last record.

page_zip_fields_decode(): Correctly decode variable-length fields having
maximum length greater than 255 bytes.

page_zip_decompress(): Remove the special treatment of the case !n_dense.
Replace the Z_NO_FLUSH argument of inflate() with Z_SYNC_FLUSH.
Treat Z_OK return value the same as Z_BUF_ERROR: always check avail_out.
After decompressing all records, decompress any trailing garbage.