read_pptx() fails for .pptx file with slide master but w/o layouts or slides

[markheckmann]

When reading a file with a slide master but without layouts or slides, read_pptx() throws an error.

Such an empty file is a bit unusual, still the error could be more informative.

File: test-no-layouts.pptx

 x <- read_pptx(file)
Error in fix.by(by.x, x) : 'by' must specify valid column