[libpng17] Restored a test on width that was removed from png.c at libpng-1.6.9

(Bug report by Alex Eubanks).
Fixed an overflow in png_combine_row with very wide interlaced images.
[libpng17] Corrected the width limit calculation in png_check_IHDR().
Merge branch 'libpng17' of /home/jbowler/src/libpng/libpng into libpng17