pngdec: Keep the input state in reset()
pngdec: Keep the input state in reset()

It's still valid after a flush and we might not get a new one.

Distrotech/gst-plugins-good,Distrotech/gst-plugins-good,Distrotech/gst-plugins-good,Distrotech/gst-plugins-good,Distrotech/gst-plugins-good