Don't panic on a null ban

like ban(req.http.non-existing-header)

bin/varnishd/cache/cache_vrt.c

@@ -416,7 +416,11 @@ VRT_ban_string(VRT_CTX, const char *str)
 
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
 	AN(ctx->vsl);
-	AN(str);
+
+	if (str == NULL) {
+		VSLb(ctx->vsl, SLT_VCL_Error, "ban(): Null argument");
+		return;
+	}
 
 	bp = BAN_Build();
 	if (bp == NULL) {

bin/varnishtest/tests/v00011.vtc

@@ -11,6 +11,7 @@ server s1 {
 varnish v1 -vcl+backend {
 	sub vcl_recv {
 		if (req.method == "PURGE") {
+			ban(req.http.doesntexist);
 			ban("");
 			ban("req.url");
 			ban("req.url //");
@@ -31,6 +32,7 @@ client c1 {
 } -run
 
 logexpect l1 -v v1 -d 1 -g vxid {
+	expect * 1004	VCL_Error {ban[(][)]: Null argument}
 	expect * 1004	VCL_Error {ban[(][)]: No ban conditions found[.]}
 	expect * 1004	VCL_Error {ban[(][)]: Expected comparison operator[.]}
 	expect * 1004	VCL_Error {ban[(][)]: Expected second operand[.]}