qemu_opt_foreach: Fix crasher

Program terminated with signal SIGSEGV, Segmentation fault.
#0  0x000055baf6ab4adc in qemu_opt_foreach (opts=0x0, func=0x55baf696b650 <net_vhost_chardev_opts>, opaque=0x7ffc51368c00, errp=0x7ffc51368e48) at util/qemu-option.c:617
617         QTAILQ_FOREACH(opt, &opts->head, next) {
[Current thread is 1 (Thread 0x7f1d4970bb40 (LWP 6603))]
(gdb) bt
#0  0x000055baf6ab4adc in qemu_opt_foreach (opts=0x0, func=0x55baf696b650 <net_vhost_chardev_opts>, opaque=0x7ffc51368c00, errp=0x7ffc51368e48) at util/qemu-option.c:617
qemu#1  0x000055baf696b7da in net_vhost_parse_chardev (opts=0x55baf8ff9260, errp=0x7ffc51368e48) at net/vhost-user.c:314
qemu#2  0x000055baf696b985 in net_init_vhost_user (netdev=0x55baf8ff9250, name=0x55baf879d270 "hostnet2", peer=0x0, errp=0x7ffc51368e48) at net/vhost-user.c:360
qemu#3  0x000055baf6960216 in net_client_init1 (object=0x55baf8ff9250, is_netdev=true, errp=0x7ffc51368e48) at net/net.c:1051
qemu#4  0x000055baf6960518 in net_client_init (opts=0x55baf776e7e0, is_netdev=true, errp=0x7ffc51368f00) at net/net.c:1108
qemu#5  0x000055baf696083f in netdev_add (opts=0x55baf776e7e0, errp=0x7ffc51368f00) at net/net.c:1186
qemu#6  0x000055baf69608c7 in qmp_netdev_add (qdict=0x55baf7afaf60, ret=0x7ffc51368f50, errp=0x7ffc51368f48) at net/net.c:1205
qemu#7  0x000055baf6622135 in handle_qmp_command (parser=0x55baf77fb590, tokens=0x7f1d24011960) at /path/to/qemu.git/monitor.c:3978
qemu#8  0x000055baf6a9d099 in json_message_process_token (lexer=0x55baf77fb598, input=0x55baf75acd20, type=JSON_RCURLY, x=113, y=19) at qobject/json-streamer.c:105
qemu#9  0x000055baf6abf7aa in json_lexer_feed_char (lexer=0x55baf77fb598, ch=125 '}', flush=false) at qobject/json-lexer.c:319
qemu#10 0x000055baf6abf8f2 in json_lexer_feed (lexer=0x55baf77fb598, buffer=0x7ffc51369170 "}R\204\367\272U", size=1) at qobject/json-lexer.c:369
qemu#11 0x000055baf6a9d13c in json_message_parser_feed (parser=0x55baf77fb590, buffer=0x7ffc51369170 "}R\204\367\272U", size=1) at qobject/json-streamer.c:124
qemu#12 0x000055baf66221f7 in monitor_qmp_read (opaque=0x55baf77fb530, buf=0x7ffc51369170 "}R\204\367\272U", size=1) at /path/to/qemu.git/monitor.c:3994
qemu#13 0x000055baf6757014 in qemu_chr_be_write_impl (s=0x55baf7610a40, buf=0x7ffc51369170 "}R\204\367\272U", len=1) at qemu-char.c:387
qemu#14 0x000055baf6757076 in qemu_chr_be_write (s=0x55baf7610a40, buf=0x7ffc51369170 "}R\204\367\272U", len=1) at qemu-char.c:399
qemu#15 0x000055baf675b3b0 in tcp_chr_read (chan=0x55baf90244b0, cond=G_IO_IN, opaque=0x55baf7610a40) at qemu-char.c:2927
qemu#16 0x000055baf6a5d655 in qio_channel_fd_source_dispatch (source=0x55baf7610df0, callback=0x55baf675b25a <tcp_chr_read>, user_data=0x55baf7610a40) at io/channel-watch.c:84
qemu#17 0x00007f1d3e80cbbd in g_main_context_dispatch () from /usr/lib64/libglib-2.0.so.0
qemu#18 0x000055baf69d3720 in glib_pollfds_poll () at main-loop.c:213
qemu#19 0x000055baf69d37fd in os_host_main_loop_wait (timeout=126000000) at main-loop.c:258
qemu#20 0x000055baf69d38ad in main_loop_wait (nonblocking=0) at main-loop.c:506
qemu#21 0x000055baf676587b in main_loop () at vl.c:1908
qemu#22 0x000055baf676d3bf in main (argc=101, argv=0x7ffc5136a6c8, envp=0x7ffc5136a9f8) at vl.c:4604
(gdb) p opts
$1 = (QemuOpts *) 0x0

The crash occurred when I tried to attach vhost-user net:

{"execute":"chardev-add","arguments":{"id":"charnet2","backend":{"type":"socket","data":{"addr":{"type":"unix","data":{"path":"/var/run/openvswitch/vhost-user1"}},"wait":false,"server":false}}},"id":"libvirt-19"}
{"return": {}, "id": "libvirt-19"}
{"execute":"netdev_add","arguments":{"type":"vhost-user","chardev":"charnet2","id":"hostnet2"},"id":"libvirt-20"}

2016-08-16 14:52:18.274+0000: 6456: error : qemuMonitorIORead:586 : Unable to read from monitor: Connection reset by peer

The solution is to teach qemu_opt_foreach() to take a shortcut if
@opts is NULL.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>

util/qemu-option.c

@@ -614,6 +614,11 @@ int qemu_opt_foreach(QemuOpts *opts, qemu_opt_loopfunc func, void *opaque,
     QemuOpt *opt;
     int rc;
 
+    if (!opts) {
+        /* Done, trivially. */
+        return 0;
+    }
+
     QTAILQ_FOREACH(opt, &opts->head, next) {
         rc = func(opaque, opt->name, opt->str, errp);
         if (rc) {