This bug was reported by Danilo Ramos of Eideticom, Inc. It has
lain in wait 13 years before being found! The bug was introduced
in zlib 1.2.2.2, with the addition of the Z_FIXED option. That
option forces the use of fixed Huffman codes. For rare inputs with
a large number of distant matches, the pending buffer into which
the compressed data is written can overwrite the distance symbol
table which it overlays. That results in corrupted output due to
invalid distances, and can result in out-of-bound accesses,
crashing the application.

The fix here combines the distance buffer and literal/length
buffers into a single symbol buffer. Now three bytes of pending
buffer space are opened up for each literal or length/distance
pair consumed, instead of the previous two bytes. This assures
that the pending buffer cannot overwrite the symbol table, since
the maximum fixed code compressed length/distance is 31 bits, and
since there are four bytes of pending space for every three bytes
of symbol space.

In addition, there is not sufficient gain from the inflate
assembler code to warrant its inclusion.

Allegedly the behavior of memcpy() is undefined if the source
pointer is NULL, even if the number of bytes to copy is zero.

This isn't the right type anyway to assure that it contains a
pointer. That type would be intptr_t or uintptr_t. However the C99
standard says that those types are optional, so their use would not
be portable. This commit simply uses size_t or whatever configure
decided to use for size_t. That would be the same length as
ptrdiff_t, and so will work just as well. The code checks to see if
the length of the type used is the same as the length of a void
pointer, so there is already protection against the use of the
wrong type. The use of size_t (or ptrdiff_t) will almost always
work, as all modern architectures have an array size that is the
same as the pointer size. Only old segmented architectures would
have to fall back to the slower CRC-32 calculation, where the
amount of memory that can be accessed is larger than the maximum
array size.

If zlib and/or gzip header processing was requested, but a header
was never provided and inflateSync was used successfully, then the
inflate state would be inconsistent, trying to compute a check
value but with no flags set. This commit sets the inflate mode to
raw in this case, since there is no other assumption that can be
made if a header was requested but never seen.

Seeing a few percent speedup by using a pointer instead of an
assigned structure. This seems to help the compiler to optimize
better.

This is a problem in the odd case that the second argument of
LSEEK is a larger type than off_t. Apparently MinGW defines off_t
to be 32 bits, but _lseeki64 has a 64-bit second argument.

Also undo a previous commit to permit MinGW to use _lseeki64.

As it is used in deflateParams().

This limits hash table inserts to the available data in the window
and to the sliding window size in deflate_stored(). The hash table
inserts are deferred until deflateParams() switches to a non-zero
compression level.

This commit allows a parameter change even if the input data has
not all been compressed and copied to the application output
buffer, so long as all of the input data has been compressed to
the internal pending output buffer. This also allows an immediate
deflateParams change so long as there have been no deflate calls
since initialization or reset.

This permits deflateParams to change the strategy and level right
after deflateInit, without having to wait until a header has been
written. The parameters can be changed immediately up until the
first deflate call that consumes any input data.

This avoids unnecessary filling of bytes in the sliding window
buffer when switching from level zero to a non-zero level. This
also provides a consistent indication of deflate having taken
input for a later commit ...

And some cosmetic cleanups.

However this ends up not really being solo, since it has to
include external libraries.

There have been many reports of bugs in the assembler codes
intended to speed up deflate and inflate. They are third-party
contributions in contrib, and so are not supported by the zlib
maintainers.