make fgetwc set error indicator for stream on encoding errors

this is a requirement in POSIX that's omitted, and seemed potentially
non-conforming, in the C standard. as such it was omitted here.
however, as part of Austin Group issue #1170, the discrepancy was
raised with WG14 and determined to be unintended; future versions of
the C standard will require the error indicator to be set, as POSIX
does.

src/stdio/fgetwc.c

@@ -25,12 +25,18 @@ static wint_t __fgetwc_unlocked_internal(FILE *f)
 	do {
 		b = c = getc_unlocked(f);
 		if (c < 0) {
-			if (!first) errno = EILSEQ;
+			if (!first) {
+				f->flags |= F_ERR;
+				errno = EILSEQ;
+			}
 			return WEOF;
 		}
 		l = mbrtowc(&wc, (void *)&b, 1, &st);
 		if (l == -1) {
-			if (!first) ungetc(b, f);
+			if (!first) {
+				f->flags |= F_ERR;
+				ungetc(b, f);
+			}
 			return WEOF;
 		}
 		first = 0;