cadence_uart: Protect against transmit errors

If qemu_chr_fe_write() returns an error (represented by a negative
number) we should skip incrementing the count and initiating a
memmove().

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Reported-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 667e5dc534d33338fcfc2471e5aa32fe7cbd13dc.1466546703.git.alistair.francis@xilinx.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

hw/char/cadence_uart.c

@@ -288,8 +288,11 @@ static gboolean cadence_uart_xmit(GIOChannel *chan, GIOCondition cond,
     }
 
     ret = qemu_chr_fe_write(s->chr, s->tx_fifo, s->tx_count);
-    s->tx_count -= ret;
-    memmove(s->tx_fifo, s->tx_fifo + ret, s->tx_count);
+
+    if (ret >= 0) {
+        s->tx_count -= ret;
+        memmove(s->tx_fifo, s->tx_fifo + ret, s->tx_count);
+    }
 
     if (s->tx_count) {
         int r = qemu_chr_fe_add_watch(s->chr, G_IO_OUT|G_IO_HUP,