io-png: Don't leak the png_infop in begin_load
io-png: Don't leak the png_infop in begin_load

If lc->png_info_ptr is not NULL, then not passing it to
png_destroy_read_struct will not release some resources.

Fallout from e8d0d8ed3d33ee6cedb75a394c36af3312b310ff

https://gitlab.gnome.org/GNOME/gdk-pixbuf/merge_requests/16

GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf