Handle PDF processor being NULL.
Handle PDF processor being NULL.

This fixes Coverity CID 282856 and 282857. This also allows the PDF
from bug 703060 to be processed by mutool clean -c without crashing.

ArtifexSoftware/mupdf,ArtifexSoftware/mupdf,ArtifexSoftware/mupdf,ArtifexSoftware/mupdf,ArtifexSoftware/mupdf,ArtifexSoftware/mupdf,ArtifexSoftware/mupdf,ArtifexSoftware/mupdf