image/png: add Fuzz function
image/png: add Fuzz function

Add a Fuzz function to package png, under the gofuzz build
tag. This function is based on the png/png.go code, from
github.com/dvyukov/go-fuzz-corpus, modified to use direct
comparison of image bounds rather than reflect.DeepEqual.

Updates #30979
Updates #19109

Change-Id: Idb86e7ded0c2d78e6cadbeda84c7b1f35b8c579c
Reviewed-on: https://go-review.googlesource.com/c/go/+/168558
Reviewed-by: thepudds <9dc9f912230dac4d6eb58b94d35b5d9394707c7a@gmail.com>
Reviewed-by: Dmitry Vyukov <05e8181be3150e47da8fe2e1f18d381c75496ff4@google.com>
Run-TryBot: Dmitry Vyukov <05e8181be3150e47da8fe2e1f18d381c75496ff4@google.com>

nawawi/go,golang/go,cloudflare/go,CAFxX/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,CAFxX/go,CAFxX/go,josharian/go,akutz/go,cloudflare/go,nawawi/go,akutz/go,nawawi/go,cloudflare/go,golang/go,nawawi/go,nawawi/go,golang/go,cloudflare/go,nawawi/go,josharian/go,golang/go,nawawi/go,josharian/go,CAFxX/go,josharian/go,golang/go,akutz/go,josharian/go,CAFxX/go,akutz/go,cloudflare/go,josharian/go,cloudflare/go,akutz/go,akutz/go,akutz/go,josharian/go,golang/go,akutz/go,CAFxX/go,CAFxX/go,golang/go,golang/go,josharian/go