Previously, using incorrect syntax for one of image, iframe, or video
functions caused an index out of range panic. Add a check to prevent
the panic and return an error instead.
Fixesgolang/go#35142
Change-Id: Ifffb4cc5daded5331d617a3db7cad84e37abadc8
Reviewed-on: https://go-review.googlesource.com/c/tools/+/203477
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>