util: Fix missing initializer for agent

In virPolkitAgentCreate neglected to initialize agent to NULL. If
there was an error in the pipe, then we jump to error and would have
an issue. Found by coverity.

src/util/virpolkit.c

@@ -166,7 +166,7 @@ virPolkitAgentDestroy(virPolkitAgentPtr agent)
 virPolkitAgentPtr
 virPolkitAgentCreate(void)
 {
-    virPolkitAgentPtr agent;
+    virPolkitAgentPtr agent = NULL;
     virCommandPtr cmd = virCommandNewArgList(PKTTYAGENT, "--process", NULL);
     int pipe_fd[2] = {-1, -1};
     struct pollfd pollfd;