util: Cleanup error path for virPolkitAgentCreate

More fallout from changing to using virPolkitAgent and handling error
paths.  Needed to clear the 'cmd' once stored and of course add the
virCommandFree(cmd) in the error: label.

src/util/virpolkit.c

@@ -182,6 +182,7 @@ virPolkitAgentCreate(void)
     if (VIR_ALLOC(agent) < 0)
         goto error;
     agent->cmd = cmd;
+    cmd = NULL;
 
     virCommandAddArgFormat(cmd, "%lld", (long long int) getpid());
     virCommandAddArg(cmd, "--notify-fd");
@@ -206,6 +207,7 @@ virPolkitAgentCreate(void)
     VIR_FORCE_CLOSE(pipe_fd[0]);
     VIR_FORCE_CLOSE(pipe_fd[1]);
     virPolkitAgentDestroy(agent);
+    virCommandFree(cmd);
     return NULL;
 }