filechooser: Fix crash when file has no content-type

This might happen for slow filesystems where a fast-content-type might
be provided instead. Don't try to manipulate that content_type if it's
NULL, otherwise we'll either throw warnings (at best) or crash (at
worse).

gtk/gtkfilechooserwidget.c

@@ -5049,6 +5049,9 @@ get_type_information (GtkFileChooserWidget *impl,
   char *description;
 
   content_type = g_file_info_get_content_type (info);
+  if (!content_type)
+    goto end;
+
   switch (impl->priv->type_format)
     {
     case TYPE_FORMAT_MIME:
@@ -5066,6 +5069,7 @@ get_type_information (GtkFileChooserWidget *impl,
       g_assert_not_reached ();
     }
 
+end:
   return g_strdup ("");
 }