qemu: fix bad free

Commit bd56d0d8 could lead to freeing an uninitialized pointer:

qemu/qemu_monitor_json.c: In function 'qemuMonitorJSONGetCommandLineOptionParameters':
qemu/qemu_monitor_json.c:4284: warning: 'cmd' may be used uninitialized in this function

* src/qemu/qemu_monitor_json.c
(qemuMonitorJSONGetCommandLineOptionParameters): Initialize variable.
Signed-off-by: NEric Blake <eblake@redhat.com>

src/qemu/qemu_monitor_json.c

@@ -4281,7 +4281,7 @@ qemuMonitorJSONGetCommandLineOptionParameters(qemuMonitorPtr mon,
                                               char ***params)
 {
     int ret;
-    virJSONValuePtr cmd;
+    virJSONValuePtr cmd = NULL;
     virJSONValuePtr reply = NULL;
     virJSONValuePtr data = NULL;
     virJSONValuePtr array = NULL;