in printf, use ferror macro rather than directly inspecting flags bit

this is purely aesthetic and should not affect code generation or
functionality.

src/stdio/vfprintf.c

@@ -132,7 +132,7 @@ static void pop_arg(union arg *arg, int type, va_list *ap)
 
 static void out(FILE *f, const char *s, size_t l)
 {
-	if (!(f->flags & F_ERR)) __fwritex((void *)s, l, f);
+	if (!ferror(f)) __fwritex((void *)s, l, f);
 }
 
 static void pad(FILE *f, char c, int w, int l, int fl)
@@ -693,7 +693,7 @@ int vfprintf(FILE *restrict f, const char *restrict fmt, va_list ap)
 		f->buf_size = 0;
 		f->wpos = f->wbase = f->wend = 0;
 	}
-	if (f->flags & F_ERR) ret = -1;
+	if (ferror(f)) ret = -1;
 	f->flags |= olderr;
 	FUNLOCK(f);
 	va_end(ap2);