s390/sclpconsole: prevent char layer callback during initialization

Starting a qemu with an sclp console and pressing a key very early
can result in
"qemu-system-s390x: hw/s390x/sclpconsole.c:60: receive_from_chr_layer:
Assertion `scon->iov' failed."

Lets make sure that the init process is finished, since the iov is
allocated after CHR_EVENT_OPENED by also checking for scon->iov.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>

hw/s390x/sclpconsole.c

@@ -44,12 +44,9 @@ typedef struct SCLPConsole {
 /* Return number of bytes that fit into iov buffer */
 static int chr_can_read(void *opaque)
 {
-    int can_read;
     SCLPConsole *scon = opaque;
 
-    can_read = SIZE_BUFFER_VT220 - scon->iov_data_len;
-
-    return can_read;
+    return scon->iov ? SIZE_BUFFER_VT220 - scon->iov_data_len : 0;
 }
 
 /* Receive n bytes from character layer, save in iov buffer,