qemu: Resolve Coverity NEGATIVE_RETURNS

Coverity notes that if the virConnectListAllDomains returns a negative
value then the loop at the cleanup label that ends on numDomains will
have issues.
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>

src/qemu/qemu_driver.c

@@ -1004,9 +1004,11 @@ qemuStateStop(void)
             ret = -1;
 
  cleanup:
-    for (i = 0; i < numDomains; i++)
-        virDomainFree(domains[i]);
-    VIR_FREE(domains);
+    if (domains) {
+        for (i = 0; i < numDomains; i++)
+            virDomainFree(domains[i]);
+        VIR_FREE(domains);
+    }
     VIR_FREE(flags);
     virObjectUnref(conn);
     virObjectUnref(cfg);