Don't segfault on trailing NULL

Author: drwilco

src/tests/urlcode01.vtc

@@ -33,6 +33,7 @@ varnish v1 -vcl+backend {
 			+ beresp.http.nonexistant + "world") + "!";
 		set beresp.http.Baz6 = urlcode.decode({"hello%20"}
 			+ beresp.http.nonexistant + "world") + "!";
+		set beresp.http.Baz7 = urlcode.decode(beresp.http.nonexistant) + "!";
 	}
 } -start
 
@@ -53,4 +54,5 @@ client c1 {
 	expect resp.http.baz5 == "hello world!"
 	expect resp.http.bar6 == "hello%20world!"
 	expect resp.http.baz6 == "hello world!"
+	expect resp.http.baz7 == "!"
 } -run

src/vmod_urlcode.c

@@ -26,9 +26,7 @@ vmod_encode(struct sess *sp, const char *str, ...)
 	e += u;
 	va_start(ap, str);
 	while (b < e && str != vrt_magic_string_end) {
-		while (str == NULL)
-			str = va_arg(ap, const char *);
-		while (b < e && *str) {
+		while (b < e && str && *str) {
 			if (visalnum((int) *str) || *str == '-' || *str == '.' 
 			    || *str ==  '_' || *str ==  '~') { /* RFC3986 2.3 */
 				*b++ = *str++;