gtkcssanimatedstyle: Fix return of new_advance()
Fix return value when gtk_css_animated_style_new_advance is called in the same frame of gtk_css_animated_style_new.
Closes #4426 (closed)
Fix return value when gtk_css_animated_style_new_advance is called in the same frame of gtk_css_animated_style_new.
Closes #4426 (closed)