util: identity: Harden virIdentitySetCurrent()

Don't unref the old identity unless we set the new one correctly and
unref the new one on failure to set it so that we don't leak any
references or use invalid pointers.

(cherry picked from commit ad886fa6)

src/util/viridentity.c

@@ -111,15 +111,17 @@ int virIdentitySetCurrent(virIdentityPtr ident)
         return -1;
 
     old = virThreadLocalGet(&virIdentityCurrent);
-    virObjectUnref(old);
 
     if (virThreadLocalSet(&virIdentityCurrent,
                           virObjectRef(ident)) < 0) {
         virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
                        _("Unable to set thread local identity"));
+        virObjectUnref(ident);
         return -1;
     }
 
+    virObjectUnref(old);
+
     return 0;
 }