[pdftex] Use of uninitialized memory when writing SMask inwrite_png_rgb_alpha

[Akira Kakuto]

Dear David,

> I think the bug is in texk/web2c/pdftexdir/writepng.c, where a "/ 2"
> should be "/ 4"; i.e., 1 in 4 bytes is an alpha byte:

Thank you very much for your report.
I changed "/ 2" as "/ 4" in r43637 in TeX Live svn.

Best,
Akira