vtc_server: Fix -Wclobbered variable during dispatch

Spotted by GCC's fortification level 3.

bin/varnishtest/tests/a00023.vtc

+varnishtest "Run server -dispatch more than once"
+
+feature ignore_unknown_macro
+
+shell {
+	cat <<-'EOF' >_.vtc
+	varnishtest "Run server -dispatch more than once (nested)"
+
+	server s0 {
+		rxreq
+		txresp
+	} -dispatch
+
+	client c1 -connect ${s0_sock} {
+		txreq
+		rxresp
+	} -run
+
+	server s0 -break
+	server s0 -dispatch
+
+	client c1 -run
+	EOF
+
+	varnishtest -v _.vtc >_.log
+}
+
+shell -match "s1 +rxhdr" { cat _.log }
+shell -match "s2 +rxhdr" { cat _.log }

bin/varnishtest/vtc_server.c

@@ -336,7 +336,8 @@ static void *
 server_dispatch_thread(void *priv)
 {
 	struct server *s, *s2;
-	int sn = 1, fd;
+	static int sn = 1;
+	int fd;
 	char snbuf[8];
 	struct vtclog *vl;
 	struct sockaddr_storage addr_s;