handle error case where the descendant font for a type 0 composite font is not a dictionary

src/UglyToad.Pdf/Fonts/Parser/Handlers/Type0FontHandler.cs

@@ -41,6 +41,10 @@
                 {
                     ParseDescendant(descendantFontDictionary, reader, isLenientParsing);
                 }
+                else
+                {
+                    throw new InvalidFontFormatException("Expected to find a Descendant Font dictionary, instead it was: " + parsed);
+                }
             }
             else
             {