Fix ucs4le_internal_loop in error case. [BZ #19726]

When converting from UCS4LE to INTERNAL, the input-value is checked for a too
large value and the iconv() call sets errno to EILSEQ. In this case the inbuf
argument of the iconv() call should point to the invalid character, but it
points to the beginning of the inbuf.
Thus this patch updates the pointers inptrp and outptrp before returning in
this error case.

This patch also adds a new testcase for this issue.
The new test was tested on a s390, power, intel machine.

ChangeLog:

	[BZ #19726]
	* iconv/gconv_simple.c (ucs4le_internal_loop): Update inptrp and
	outptrp in case of an illegal input.
	* iconv/tst-iconv6.c: New file.
	* iconv/Makefile (tests): Add tst-iconv6.