Fix crashes reported by Bennet Helm and Kornel Benko

Fix crashes reported by Bennet Helm and Kornel Benko
	* src/support/unicode.[Ch]
	(IconvProcessor): Rework the implementation to fix two bugs:
	- iconv_close was called even if the conversion descriptor was
	  invalid
	- The compiler generated copy constructor did simply copy the pimpl_
	  pointer. Therefore pimpl_ was deleted twice for all IconvProcessor
	  instances created by eightbit_to_ucs4() and ucs4_to_eightbit().
	  This is solved by using a scoped_ptr for pimpl_.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@15937 a592a061-630c-0410-9148-cb99ea01b6c8