src/img2pdf.py: handle Pillow returning a DPI of zero (closes: #36)

src/img2pdf.py

@@ -600,6 +600,13 @@ def get_imgmetadata(imgdata, imgformat, default_dpi, colorspace, rawdata=None):
         ndpi = (int(round(ndpi[0])), int(round(ndpi[1])))
         ics = imgdata.mode
 
+    # Since commit 07a96209597c5e8dfe785c757d7051ce67a980fb or release 4.1.0
+    # Pillow retrieves the DPI from EXIF if it cannot find the DPI in the JPEG
+    # header. In that case it can happen that the horizontal and vertical DPI
+    # are set to zero.
+    if ndpi == (0, 0):
+        ndpi = (default_dpi, default_dpi)
+
     logging.debug("input dpi = %d x %d", *ndpi)
 
     if colorspace: