Increase a bit the robustness of mat2

Those issues were found via https://github.com/google/atheris

libmat2/audio.py

@@ -13,7 +13,8 @@ class MutagenParser(abstract.AbstractParser):
     def __init__(self, filename):
         super().__init__(filename)
         try:
-            mutagen.File(self.filename)
+            if mutagen.File(self.filename) is None:
+                raise ValueError
         except mutagen.MutagenError:
             raise ValueError
 

libmat2/images.py

@@ -26,7 +26,10 @@ class SVGParser(exiftool.ExiftoolParser):
                       }
 
     def remove_all(self) -> bool:
+        try:
             svg = Rsvg.Handle.new_from_file(self.filename)
+        except GLib.GError:
+            raise ValueError
         dimensions = svg.get_dimensions()
         surface = cairo.SVGSurface(self.output_filename,
                                    dimensions.height,