According to the zlib documentation, next_in and avail_in
must be initialized before invoking inflateInit or inflateInit2.
Furthermore, the zalloc function must clear the allocated memory.

btr_copy_zblob_prefix(): Replace the d_stream parameter with buf,len
and return the copied length.

page_zip_decompress(): Invoke inflateInit2 a little later.

page_zip_zalloc(): Rename from page_zip_alloc().
Invoke mem_heap_zalloc() instead of mem_heap_alloc().

rb:619 approved by Jimmy Yang