also ensure that write buffer is bounded when __stdio_write returns

assuming other code is correct, this should be a no-op, but better to
be safe...

src/stdio/__stdio_write.c

@@ -22,6 +22,7 @@ size_t __stdio_write(FILE *f, const unsigned char *buf, size_t len)
 		cnt = syscall_cp(SYS_writev, f->fd, iov, iovcnt);
 		pthread_cleanup_pop(0);
 		if (cnt == rem) {
+			f->wend = f->buf + f->buf_size;
 			f->wpos = f->wbase = f->buf;
 			return len;
 		}