libc/locale: Check EILSEQ return

_citrus_iconv_std_iconv_convert can return EILSEQ without setting
invlen. As a result, we should check EILSEQ in the return value or it
will incorrectly return -2

Reviewed by:    kevans
Fixes: e2030ca246 ("libc: fix c*rtomb/mbrtoc*")
MFC after:      2 days
Differential Revision: https://reviews.freebsd.org/D61151

lib/libc/locale/mbrtocXX_iconv.h

@@ -123,7 +123,7 @@ mbrtocXX_l(charXX_t * __restrict pc, const char * __restrict s, size_t n,
 			continue;
 
 		/* Illegal sequence. */
-		if (invlen > 0) {
+		if (invlen > 0 || err == EILSEQ) {
 			cs->srcbuf_len = 0;
 			errno = EILSEQ;
 			return ((size_t)-1);