virtio-9p: Avoid SEGV when log file couldn't be opened

While running in debug mode if 9P server is unable to open the log file
it results in a SEGV deep down in glibc:

Program received signal SIGSEGV, Segmentation fault.
0x008fca8c in fwrite () from /lib/libc.so.6
(gdb) bt
#0  0x008fca8c in fwrite () from /lib/libc.so.6
#1  0x081eb87e in pprint_pdu (pdu=0x89a52e1c)
    at /data/sripathi/code/qemu/new/qemu-next-upstream/hw/virtio-9p-debug.c:380
#2  0x0806dad8 in submit_pdu (s=0x897dc008, pdu=0x89a52e1c)
    at /data/sripathi/code/qemu/new/qemu-next-upstream/hw/virtio-9p.c:3092
#3  0x0806dc63 in handle_9p_output (vdev=0x897dc008, vq=0x86d8218)
    at /data/sripathi/code/qemu/new/qemu-next-upstream/hw/virtio-9p.c:3122
#4  0x081ac728 in virtio_queue_notify (vdev=0x897dc008, n=0)
    at /data/sripathi/code/qemu/new/qemu-next-upstream/hw/virtio.c:563
#5  0x08063876 in virtio_ioport_write (opaque=0x86d7b98, addr=16, val=0)
    at /data/sripathi/code/qemu/new/qemu-next-upstream/hw/virtio-pci.c:222
#6  0x08063e26 in virtio_pci_config_writew (opaque=0x86d7b98, addr=16, val=0)
    at /data/sripathi/code/qemu/new/qemu-next-upstream/hw/virtio-pci.c:357
#7  0x080c881a in ioport_write (index=1, address=49296, data=0) at ioport.c:80
#8  0x080c8d4c in cpu_outw (addr=49296, val=0) at ioport.c:204
#9  0x08073010 in kvm_handle_io (port=49296, data=0xab393000, direction=1, size=2, count=1)
    at /data/sripathi/code/qemu/new/qemu-next-upstream/kvm-all.c:735
...
...

This is ugly and misleading. The following patch adds a BUG_ON to catch this
error. With this patch we get an abort message like the following, which makes
it easier to analyze:

f12-kvm login: qemu: /data/sripathi/code/qemu/new/qemu-next-upstream/hw/virtio-9p-debug.c:353: pprint_pdu: Assertion `!(!llogfile)' failed.
Signed-off-by: NSripathi Kodi <sripathik@in.ibm.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

hw/virtio-9p-debug.c

@@ -327,6 +327,8 @@ void pprint_pdu(V9fsPDU *pdu)
         llogfile = fopen("/tmp/pdu.log", "w");
     }
 
+    BUG_ON(!llogfile);
+
     switch (pdu->id) {
     case P9_TVERSION:
         fprintf(llogfile, "TVERSION: (");