fix treatment by fgetws of encoding errors as eof

fgetwc does not set the stream's error indicator on encoding errors,
making ferror insufficient to distinguish between error and eof
conditions. feof is also insufficient, since it will return true if
the file ended with a partial character encoding error.

whether fgetwc should be setting the error indicator itself is a
question with conflicting answers. the POSIX text for the function
states it as a requirement, but the ISO C text seems to require that
it not. this may be revisited in the future based on the outcome of
Austin Group issue #1170.

src/stdio/fgetws.c

 #include "stdio_impl.h"
 #include <wchar.h>
+#include <errno.h>
 
 wint_t __fgetwc_unlocked(FILE *);
 
@@ -11,6 +12,10 @@ wchar_t *fgetws(wchar_t *restrict s, int n, FILE *restrict f)
 
 	FLOCK(f);
 
+	/* Setup a dummy errno so we can detect EILSEQ. This is
+	 * the only way to catch encoding errors in the form of a
+	 * partial character just before EOF. */
+	errno = EAGAIN;
 	for (; n; n--) {
 		wint_t c = __fgetwc_unlocked(f);
 		if (c == WEOF) break;
@@ -18,7 +23,7 @@ wchar_t *fgetws(wchar_t *restrict s, int n, FILE *restrict f)
 		if (c == '\n') break;
 	}
 	*p = 0;
-	if (ferror(f)) p = s;
+	if (ferror(f) || errno==EILSEQ) p = s;
 
 	FUNLOCK(f);