Hi,

I am using pdfcpu (latest version from GitHub cloned) on a Mac to fill a PDF with some textboxes (later via API in an application).

If I add content to one of the fields, pdfcpu crashes:

writing out.pdf...
unexpected panic attack: runtime error: slice bounds out of range [1:0]

exit status 1

The slice in question is in this line and selecting a fontID. When I provide a default value like

fontID = "Helv"
if len(da[i-2]) > 1 {
	fontID = da[i-2][1:]
}

the pdf is filled successfully.

It would be great if pdfcpu would either give a better error message (like PDF is corrupt with no fontID) or have a default behavior (with warning).

Thanks for this awesome project!