setvbuf: return failure if mode is invalid

POSIX requires setvbuf to return non-zero if `mode` is not one of _IONBF,
_IOLBF, or _IOFBF.

src/stdio/setvbuf.c

@@ -12,13 +12,15 @@ int setvbuf(FILE *restrict f, char *restrict buf, int type, size_t size)
 
 	if (type == _IONBF) {
 		f->buf_size = 0;
-	} else {
+	} else if (type == _IOLBF || type == _IOFBF) {
 		if (buf && size >= UNGET) {
 			f->buf = (void *)(buf + UNGET);
 			f->buf_size = size - UNGET;
 		}
 		if (type == _IOLBF && f->buf_size)
 			f->lbf = '\n';
+	} else {
+		return -1;
 	}
 
 	f->flags |= F_SVB;