qemu-ga: Fix null pointer passed to unlink in failure branch

Clang reports this warning:

Null pointer passed as an argument to a 'nonnull' parameter
Reviewed-by: NLuiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>
(cherry picked from commit 4bdb1a30)
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

qemu-ga.c

@@ -439,7 +439,9 @@ static void become_daemon(const char *pidfile)
     return;
 
 fail:
-    unlink(pidfile);
+    if (pidfile) {
+        unlink(pidfile);
+    }
     g_critical("failed to daemonize");
     exit(EXIT_FAILURE);
 #endif