[libpng17] Added an "assert()" to pngfix.c to quiet a clang scanner warning.
[libpng17] Added another "assert()" to pngfix.c
[libpng17] Imported from libpng-1.7.0beta23.tar
[libpng17] Bump version to 1.7.0beta24
[libpng17] Updated CHANGES and ANNOUNCE