Added a check for broken symlinks, should fix probonopd#210

[dandreossi]

If in xdg.xdg.DataHome + "/applications/" is present an invalid symlink, the appimaged wrapper crash, often without starting the application. Issue should be solved by adding a check if the symlink is valid and, if it's valid, the target file will be parsed.

[probonopd]

Thanks @dandreossi