Open a pdf(view) link of a djvu file (mistakenly) causes severe memory allocation

[firmart]

I just found out an issue comes from org-pdfview inherited by org-pdftools. If one mistakenly opens a link [[pdf:xxxx.djvu][DJVU file]], it causes severe memory allocation and freezes the PC. Since djvu, ps or pdf are all somehow usual, I think it's better to check in the open function if it really deals with a pdf before trying open it.

It's a bit annoying to accidentally make the mistake and make the PC freeze.