qemu: Fix crash in migration of graphics-less guests.

Commit 7f15ebc7 introduced a bug
happening when guests without a <graphics> element are migrated.
The initialization of listenAddress happens unconditionally
from the cookie even if the cookie->graphics pointer was NULL.
Moved the initialization to where it is safe.
Signed-off-by: NViktor Mihajlovski <mihajlov@linux.vnet.ibm.com>

src/qemu/qemu_migration.c

@@ -1723,7 +1723,7 @@ qemuDomainMigrateGraphicsRelocate(virQEMUDriverPtr driver,
 {
     qemuDomainObjPrivatePtr priv = vm->privateData;
     int ret;
-    char *listenAddress = cookie->graphics->listen;
+    char *listenAddress;
 
     if (!cookie)
         return 0;
@@ -1737,6 +1737,7 @@ qemuDomainMigrateGraphicsRelocate(virQEMUDriverPtr driver,
     if (cookie->graphics->type != VIR_DOMAIN_GRAPHICS_TYPE_SPICE)
         return 0;
 
+    listenAddress = cookie->graphics->listen;
     if (!listenAddress ||
         STREQ(listenAddress, "0.0.0.0") ||
         STREQ(listenAddress, "::"))