Fix buffer mux handling for unconnected serial ports


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@3737 c046a42c-6fe2-441c-8c8c-71466251a162
diff --git a/hw/slavio_serial.c b/hw/slavio_serial.c
index 9a8e340..6705d44 100644
--- a/hw/slavio_serial.c
+++ b/hw/slavio_serial.c
@@ -475,7 +475,8 @@
         else
             ret = s->rx;
         SER_DPRINTF("Read channel %c, ch %d\n", CHN_C(s), ret);
-        qemu_chr_accept_input(s->chr);
+        if (s->chr)
+            qemu_chr_accept_input(s->chr);
         return ret;
     default:
         break;