iodev: Do not print leading * for empty buffer prints on alt path

Signed-off-by: Hector Martin <marcan@marcan.st>

src/iodev.c

@@ -83,10 +83,12 @@ int in_iodev = 0;
 void iodev_console_write(const void *buf, size_t length)
 {
     if (in_iodev || !is_primary_core()) {
+        if (length) {
             iodev_write(IODEV_UART, "*", 1);
             iodev_write(IODEV_UART, buf, length);
             return;
         }
+    }
     in_iodev++;
 
     dprintf("  iodev_console_write() wp=%d\n", con_wp);