Memory leak: free conn->retbuf in uxagentc.c.

While debugging some new code, I ran valgrind in leak-checking mode
and it pointed out a handful of existing memory leaks, which got in the
way of spotting any _new_ leaks I might be introducing :-)

This was one: in the case where an asynchronous agent query on Unix is
aborted, the dynamically allocated buffer holding the response was not
freed.

unix/uxagentc.c

@@ -95,6 +95,8 @@ void agent_cancel_query(agent_pending_query *conn)
     uxsel_del(conn->fd);
     close(conn->fd);
     del234(agent_pending_queries, conn);
+    if (conn->retbuf && conn->retbuf != conn->sizebuf)
+        sfree(conn->retbuf);
     sfree(conn);
 }
 
@@ -114,11 +116,12 @@ static void agent_select_result(int fd, int event)
 	return;		       /* more data to come */
 
     /*
-     * We have now completed the agent query. Do the callback, and
-     * clean up. (Of course we don't free retbuf, since ownership
-     * of that passes to the callback.)
+     * We have now completed the agent query. Do the callback.
      */
     conn->callback(conn->callback_ctx, conn->retbuf, conn->retlen);
+    /* Null out conn->retbuf, since ownership of that buffer has
+     * passed to the callback. */
+    conn->retbuf = NULL;
     agent_cancel_query(conn);
 }