base: Fix loop range in pngwriter
base: Fix loop range in pngwriter

The inner loop range limit should be width instead of height.

Change-Id: I091c590713c945d4bd04ffcc974d4eb8aa23d1b2
Signed-off-by: Chun-Chen Hsu <91dcdfb9561c211c6d292d4179e3ae64e470601d@google.com>
Reviewed-on: https://gem5-review.googlesource.com/9081
Maintainer: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>
Reviewed-by: Jason Lowe-Power <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>

gem5/gem5,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,gem5/gem5,gem5/gem5,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu,gem5/gem5,gem5/gem5