Introduction of '-' CLI prefix allowed empty commands to sneak through.

Fixes #2647

bin/varnishtest/tests/r02647.vtc

+varnishtest "empty cli command"
+
+server s1 {
+	rxreq
+	txresp
+} -start
+
+varnish v1 -vcl+backend {} -start
+
+varnish v1 -clierr 100 "-"
+
+client c1 {
+	txreq
+	rxresp
+} -run

lib/libvarnish/vcli_serve.c

@@ -278,6 +278,12 @@ cls_exec(struct VCLS_fd *cfd, char * const *av)
 			break;
 		}
 
+		if (av[1] == NULL) {
+			VCLI_Out(cli, "Empty CLI command.\n");
+			VCLI_SetResult(cli, CLIS_SYNTAX);
+			break;
+		}
+
 		if (isupper(av[1][0])) {
 			VCLI_Out(cli, "all commands are in lower-case.\n");
 			VCLI_SetResult(cli, CLIS_UNKNOWN);