to report the count of unused bytes at the end correctly.  Add the
ability to provide initial input to blast().

Also correct typo for 64-bit debug build.

Without this fix, it would be possible to construct inputs to puff
that would cause it to segfault.

This patch allows zlib to compile cleanly with the -Wcast-qual gcc
warning enabled, but only if ZLIB_CONST is defined, which adds
const to next_in and msg in z_stream and in the in_func prototype.
A --const option is added to ./configure which adds -DZLIB_CONST
to the compile flags, and adds -Wcast-qual to the compile flags
when ZLIBGCCWARN is set in the environment.