fix the appnd() macro - it needs a block

when called below in

	if (ua_prepend[i])
		appnd(w, space, ua_prepend[i], l);

appnd would exand such that only the first line (l = strlen(r))
was conditional on ua_prepend[i] being non-null, which then would
trigger a null pointer dereference in strncpy.

This is a particularly embarassing bug.

src/dcs_varnish.c

@@ -100,16 +100,18 @@ const char * const ua_prepend[DCS_VARNISH2_NHDRS] = {
 	[3] = NULL
 };
 
-#define appnd(w, space, r, l)			\
-	l = strlen(r);				\
-	strncpy(w, r, space);			\
-	if (l > space) {			\
-		w += space;			\
-		space = 0;			\
-		break;				\
-	}					\
-	space -= l;				\
-	w += l;
+#define appnd(w, space, r, l)				\
+	do {						\
+		l = strlen(r);				\
+		strncpy(w, r, space);			\
+		if (l > space) {			\
+			w += space;			\
+			space = 0;			\
+			break;				\
+		}					\
+		space -= l;				\
+		w += l;				\
+	} while(0)
 
 static int
 dcs_varnish_classify(dcs_ctx *ctx) {